Friday, 16 October 2009

A long shadow is cast on the plot...

A couple of days ago, Cedric asked the question how we could add a shadow to a pm3d surface plot. In some sense, the problem turned out to be easier, than I had expected, but I am not sure that I did what he actually meant. Anyway, we could use it as our working hypothesis, and refine it, if necessary.

The idea of putting "phong" on the surface was discussed ages ago, and I won't re-open that question here. For the shadow, we will just replot our surface (defined as z(x,y)) in a little bit strange way: instead of letting the x and y run through their corresponding ranges, we will restrict y to be equal to the maximum of the yrange. By doing so, we get this
Here is our (short) script:
unset colorbox; unset key
set iso 2, 50
set parametric; set urange [0:1]; set vrange [0:1.2]
set table ''
splot u, v, 1
unset table
unset parametric
set iso 100, 100
set xrange [-3:3]
set yrange [-3:3]
set zrange [0:1.2]
set table ''
splot exp(-x*x-y*y)+0.1*rand(0)
unset table
set ticslevel 0
set pm3d
set cbrange [0:4]
f(x,y,z,a,b,s) = z*(exp(-(x-a)*(x-a)/s-(y-b)*(y-b)/s)/3.0+0.66)
set palette defined (0 "#ff2222", 1 "#ffeeee", 2 "#aaaaaa", 3 "#2222ff", 4 "#8888ff")
splot '' u ($1*6.0-3.0):(3):2:($2+3.0) w pm3d, \
'' u (-3):($1*6.0-3.0):2:($2+3.0) w pm3d, \
'' u ($1*6.0-3.0):($2*6.0-3.0):(0):(2.1) w pm3d, \
'' u ($1+1.0):(3):($3*0.7):(2.1) w pm3d,\
'' u 1:2:3:(f($1,$2,$3,-0.5,-0.5,.8)) w pm3d
First, we create the data that will be our background, then some dummy data, which in this particular case will be a noise Gaussian function in 2D. Then we move the surface to the bottom of the zrange, by setting the ticslevel to 0. The next step is the definition of the cbrange. We need to "overdefine" this, i.e., our cbrange is much larger, than the actual data range. The reason for this is that in this way, we can use the same colour palette, and we do not have resort to multiplot. The basic problem with multiplot is that we since we want to plot onto the same graph, we would have to re-set the border, tics, labels, and so on. By using the same plot, and same palette, we can avoid all this hassle. The price we pay is that our palette will be a bit more complicated, but we can live with that. Now, we have to define our palette, which will change between red and almost white for [0:1], and between blue, and almost white for [3:4]. We also defined a value at 2, which we will use for colouring the shadow, but the two main ranges are [0:1], and [3:4]. Note that we define disjoint ranges, thereby not walking into a trouble with the end points.
Finally, we plot our background, and function. Pay attention to plots like
splot '' u ($1*6.0-3.0):(3):2:($2+3.0) w pm3d
This will plot a plane between x [-3:3], and z [0:1] at y=3, with a colour given by the value of ($2+3.0). This is nothing but pushing all z values in the plot into the [3:4] colour range.

At the very end, we plot our function's shadow, namely,
'' u ($1+1.0):(3):($3*0.7):(2.1) w pm3d
where the x values are shifted by 1.0 (so as to give the impression that the surface is lit from the (-1:-1) direction), the y values are all restricted to 3, which is the maximum of the yrange, and the z values are multiplied by 0.7, again for the same reason. Colouring is done by using one single value, 2.1. The very last step is to plot the surface itself, using the colouring given by f(...). If you want to have another direction for the lighting, or have a tighter focus, you should change the parameters in this function.

Tuesday, 13 October 2009

The shiny histograms, again

Do you remember those shiny histograms that we discussed some long, long time ago, at the beginning of the summer? And do you also remember how much of a hassle it was to create them, since we relied on an external gawk script, and we had to build our rectangles, one by one? And have you ever wondered what all that fuss was about and whether there was an easier solution? A one-liner, perhaps? If the answer to these questions lies in the affirmative, go no further! We will discuss a method of making those histograms, without an external script, only with legal gnuplot commands, and in 5 lines. I understand that 5 lines is just 4 lines longer, than one would expect from a one-liner, but on the other hand, three out of those 5 lines are equivalent, so I don't feel so bad about this any more:)
OK, so here is our figure

here is our data file, which we will call 'hist.dat'
1 2 3
2 2 2
4 10 3
5 1 4
5 6 2
and here are our scripts, 'hist.gnu',
unset key; set xtics nomirror; set ytics nomirror; set border front;
div=1.1; bw = 0.9; h=1.0; BW=0.9; wd=10; LIMIT=255-wd; white = 0
red = "#080000"; green = "#000800"; blue = "#000008"
set auto x
set yrange [0:11]
set style data histogram
set style histogram cluster gap 1
set style fill solid
set boxwidth bw
set multiplot
plot 'hist.dat' u 1 lc rgb red, '' u 2 lc rgb green, '' u 3 lc rgb blue
unset border; set xtics format " "; set ytics format " "; set ylabel " "
call 'hist_r.gnu'
unset multiplot
and 'hist_r.gnu'
bw=BW*cos(white/LIMIT*pi/2.0); set boxwidth bw; white=white+wd
red = sprintf("#%02X%02X%02X", 128+white/2, white, white);
green = sprintf("#%02X%02X%02X", white, 128+white/2, white);
blue = sprintf("#%02X%02X%02X", white, white, 128+white/2);
if(white<LIMIT) reread

Then let us see what is happening here! At the beginning, we define various variables, most notably, white, red, green, and blue. The rest up to the first plot command is nothing but setting up the figure: we define the range, tell gnuplot to treat our data as histogram, set the width of the bars, and finally, set multiplot.
There is nothing exciting in the first plot, except, that we specify the colour of the bars as

plot 'hist.dat' u 1 lc rgb red, '' u 2 lc rgb green, '' u 3 lc rgb blue
The strings red, green, and blue were defined at the beginning of our first script, thus, we learn here that gnuplot will accept any defined (and valid) string as the specifier of the colour. After our first plot, we unset the border, re-set the format of the xtic and ytic to empty, and do likewise with the ylabel. Should there be an xlabel, we should have to do the same there. Having done this, we call our second script, which we will dissect now. This is really nothing but a 'for' loop, that we have discussed a couple of times before. In fact, quite a few times. The first two commands re-set the widths of the bars in the next plot. Note that I set the width in such a way that it would draw the outline of a circle as we step through the values of white. This is what we increment next, mind you!

The next three lines are basically identical: we re-define the colours, using a sprintf command in each step. If you recall how the RGB colours are defined, we have to create a string that looks like
say. This is what our sprintf command will do, returning a string of this form that depends on the value of 'white'. There are some small nuances in the corresponding colour channel of red, green, and blue, respectively, namely, that the base colour for red was
and we want to linearly interpolate between this colour, and white,
so, we have to apply the relevant linear function, but there is nothing beyond this. Obviously, if you are unhappy with the colour scheme that I have (I know full well that these are not the best colours...), this is the place where you would have to tamper with the script. When we are done with re-defining the widths, and the colours, we simply replot our histogram, and do that, as long as the value of white is smaller, than the limit that we set at the beginning. In this particular case, 245. In most cases, we do not need this many plots, by the way! For a raster plot, 10-12 steps, for a vector format, something like 15-16 steps should be more than enough.

At the end, we shouldn't forget about unsetting the multiplot. The only difficulty that I see with this figure is that it is not so straightforward to use a key. However, it is not terribly hard to come up with a solution for this problem: all we have to do is to put three vertical labels on the top of the first, second, and third column, indicating what they represent.

Monday, 12 October 2009

Putting figures into perspective

As I promised yesterday, this time, we will try to skew our figure, as if it was in 3D, and we were looking at it from an angle. I must admit that this is something that one would not put in a publication, unless it is a poster or presentation, perhaps. In those cases, however, it might lend a refreshing new, errr.., perspective to our data. Before diving into the script, here is the figure, so you can decide whether you want to read on:)

Then, here is our script that was responsible for the figure
xmin = 0; xmax = 10; zmin = -0.4; zmax = 0.75
set view 60, 30
unset key; unset colorbox
set border 1+16+128+1024
unset ytics; set xtics out nomirror; set ticslevel 0
set yrange [0:-0.1]
set zrange [zmin:zmax]
set grid front; unset grid
set xtics xmin,2,xmax-1
set ztics zmin,0.2,zmax
f(x) = exp(-x/4.0)*sin(x)
c(x) = exp(-(x-xmax/3.0)*(x-xmax/3.0)/1.0)
set xlabel 'Time [a.u.]'
set label 2 'Amplitude [a.u.]' at graph -0.35, 0.3 rotate by 90
set parametric
set iso 3, 3
set urange [xmin:xmax]
set vrange [zmin:zmax]
set table 'perspective1.dat'
splot u, 0, v
unset table
set urange [xmin+0.2:xmax-0.2]
set table 'perspective2.dat'
splot u, 0, f(u)
unset table
unset parametric
set size 1.4, 1.4
set palette defined (0 "#7b6cff", 1 "#eeeeff")
splot 'perspective1.dat' u 1:2:3:(c($1)) w pm3d, \
'perspective2.dat' u ($1+0.05):2:($3-0.02) w l lw 6 lc rgb "#888888", \
'' w l lw 6 lt 1

The trick that we use here is to plot in 3D, and take off all those elements of the figure that we do not actually need. In the beginning, we define a couple of variables, in order to make our life a bit easier. Then we unset the colorbox and the key, and set only those borders that we want to see. If you are interested in how I came up with those numbers in the definition of the border, just issue the command

in the gnuplot prompt. After this, we keep unsetting things, and define our ranges. The next noteworthy command is
set grid front; unset grid

At first, this seems silly, but the reason is real: in the figure we have a background, which would obscure our tic marks. The way out of this problem is to push the tic marks to the front, which is achieved by setting the grid to the front. Since we do not actually need the grid, we unset it, but the setting, its front position, is still there. The tic marks inherit the position of the grid, and thus, they will be in the forefront.

The function definitions are f(x), the function that we want to plot, and c(x), which will determine the colouring of our background. Modify them accordingly.

We set the zlabel by hand, because it might not be possible to turn it by 90 degrees otherwise. (Not all terminals would support it.) Then we plot the background, and the function, both to a file, so that it will be easier to colour them in the next step, where we plot the real thing. Note that in the plot of the background, we use four columns, while there are only 3 in the data file. The fourth one determines the colour as the position on the graph. The colour is given by the function c(x), and the palette, which we defined one line earlier. You should change these two things, if you are not satisfied with the background you get. Finally, we plot the function twice. Once in gray, a bit shifted to the right and down, and for the second time, in red. In this way, we add a shadow to our curve. If you want to improve the shadow, you should look at my post from the 30th August

I should also add that we discussed a vertical skew. In case you want to skew the figure horizontally, all you have got to do is to plot on the x-y plain instead of x-z.

Sunday, 11 October 2009

Adding a variable grid to a plot

Useful as it is, sometimes the standard grid is not the best way to guide the eye when reading off values in a graph. Today I would like to discuss a couple of other options, none of them standard in gnuplot. Fortunately, we haven't got to leave the realm of gnuplot, everything can be done without any external scripts. To make things even more appealing, the solution requires only a couple of lines of code.

Our first attempt will be to "project" the values of a curve to the two axes, as in this figure

The data file, helpergrid.dat, contains only a couple of lines,
1 1
4 2
9 3
16 4
25 5
36 6

and the script reads as follows
unset key
set xrange [0:40]
set yrange [0:7]
set xlabel 'Effort [a.u.]'
set ylabel 'Output [a.u.]'
p 'helpergrid.dat' u 1:2:(0):1 w xerror ps 0 lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:2 w i lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:2 w lp pt 7 ps 1.5

Drawing the vertical lines is easy, for there is a plotting style in gnuplot, impulse, which does just that. The horizontal ones are not that trivial, but take only one line of code: the idea is that we trick gnuplot into drawing the horizontal lines by something else. In this case, this something else is the error bar, in particular, the xerror bar. This is what happens in the first plotting line, after we set up the figure. The first plot calls our data file, and draws the errors that span from x=0 to x=x_data. We also specify the line type, 0, which will be dashed, the line width, and the line colour. The second plot draws the vertical lines, plotting our data with impulses. Again, we specify the line type, width, and colour, so that it conforms with the previous one. This is necessary, because, when left alone, gnuplot assigns a new line type to each new plot. Finally, we plot the data. This was simple.

I should add here that we can draw a "full" grid based on the data values. If we replace the plotting command by
p 'helpergrid.dat' u (40):2:(0):(40) w xerror ps 0 lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:(7) w i lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:2 w lp pt 7 ps 1.5

then the horizontal lines will be drawn between 0 and 40, while the vertical ones between 0 and 7. Now, it might happen that you don't know the xrange and yrange, in which case you can use the variables GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN, GPVAL_Y_MAX to specify the plot. Of course, we have to call a dummy plot beforehand. We have discussed this a couple of times. If in doubt, look up the post on how to plot a martix with bars.

Well, we have cleared the first barrier, but what if we wanted to place the numbers where the data points are? This is quite simple: we only have to plot the file twice more, this time with xticlabel and yticlabel. So, here is our script
unset key
set xrange [0:40]; set yrange [0:7]
set xlabel 'Effort [a.u.]'
set ylabel 'Output [a.u.]'
p 'helpergrid.dat' u 1:2:(0):1 w xerror ps 0 lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:2 w i lt 0 lw 0.5 lc rgb "#ff0000", \
'' u 1:2 w lp pt 7 ps 1.5, \
'' u 1:(0):xticlabel(1) w p ps 0, '' u (0):2:yticlabel(2) w p ps 0

and here is our figure

Note that in this case, if you want to produce a full grid (wall-to-wall, floor-to-ceiling), we can just set the grid, and forget about the error bars. The script for that case would be
unset key
set xrange [0:40]; set yrange [0:7]
set xlabel 'Effort [a.u.]'
set ylabel 'Output [a.u.]'
set grid lt 0 lw 0.5 lc rgb "#ff0000"
p 'helpergrid.dat' u 1:2 w lp lt 3 pt 7 ps 1.5, \
'' u 1:(0):xticlabel(1) w p ps 0, '' u (0):2:yticlabel(2) w p ps 0

and this would result in this figure.

I think what we covered today was fairly simple, but still useful. Next time I will show how figures can be skewed, yet another way of making the reader perceive a 2D plot in 3D surroundings. Till then!

Saturday, 3 October 2009

More on histograms

Someone asked me whether it would be possible to make a histogram that looks like those that MS Office can create: filled with gradient, casting a shadow, and labelled according to their value. Frankly, I just couldn't admit defeat, and I had to figure out a way. But then, it turned out to be rather simple, so I thought that I would share it here. We are going to make this figure

from this data, called 'msbar.dat'
Max 1.0 1.0 1.2
Min 0.9 0.2 0.95
Avg 0.95 0.5 1.1

I have already discussed a way to beefing up the histograms (It was titled Shiny histograms, or something similar.), but I must say, that method is a bit convoluted. The reason for this is that we used a parametric plot. We can avoid that by plotting to a file first, and then plotting the file as many times as needed. We can't save the trouble of having to read our data file, but this is rather simple. We can do that either by employing a very primitive external script, or writing the script in gnuplot, as we discussed it in, say, the post on the recession graph. Given that we haven't got to process any data, this latter method is probably less desirous. Gnuplot is not for printing lines and the like, after all.

After the introduction, let us see the script!
# First, the gradient for the bars
set xrange [0:1]; set yrange [0:1]; set isosample 2, 200
set table 'msbar_bar.dat'
splot y
unset table

# Then we make the shadow
set isosample 200, 50
set table 'msbar_bar_sh.dat'
splot (1.0-exp((x-1.0)*20.)) #*(1.0-exp((y-1.0)*20.0))
unset table

unset key; unset colorbox
xw = 0.2; set boxwidth xw; sh = 0.03
gf(x) = x*x*x*x #change this, if a tighter gradient is needed
set cbrange [0:7]
set xrange [-0.5:2.5]
set yrange [0:1.4]
set grid ytics lw 0.5 lc rgb "#868686"
set xtics nomirror
set ylabel 'Performance [a.u.]'
set palette defined (0 '#e0e8f5', 1 '#31bd71', 2 '#e0e8f5', 3 '#d99795', 4 '#e0e8f5', 5 '#9ab5e4', 6 "#ffffff", 7 "#a2a2a2")
plot 'msbar.dat' u 0:(0):xticlabel(1) w l, \
'' u ($0-xw):($2+0.1):(stringcolumn(2)) w labels, \
'' u ($0):($3+0.1):(stringcolumn(3)) w labels, \
'' u ($0+xw):($4+0.1):(stringcolumn(4)) w labels, \
'msbar_bar_sh.dat' u ($1*xw-1.5*xw+sh):($2*1.0-sh):($3+6.0) w ima, \
'' u ($1*xw-.5*xw+sh):($2*1.0-sh):($3+6.0) w ima, \
'' u ($1*xw+.5*xw+sh):($2*1.2-sh):($3+6.0) w ima, \
'' u ($1*xw-1.5*xw+sh+1):($2*0.9-sh):($3+6.0) w ima, \
'' u ($1*xw-.5*xw+sh+1):($2*0.2-sh):($3+6.0) w ima, \
'' u ($1*xw+.5*xw+sh+1):($2*0.95-sh):($3+6.0) w ima, \
'' u ($1*xw-1.5*xw+sh+2):($2*0.95-sh):($3+6.0) w ima, \
'' u ($1*xw-.5*xw+sh+2):($2*0.5-sh):($3+6.0) w ima, \
'' u ($1*xw+.5*xw+sh+2):($2*1.1-sh):($3+6.0) w ima, \
'msbar_bar.dat' u ($1*xw-1.5*xw):($2*1.0):(gf($3)) w ima, \
'' u ($1*xw-.5*xw):($2*1.0):(gf($3)+2.0) w ima, \
'' u ($1*xw+.5*xw):($2*1.2):(gf($3)+4.0) w ima, \
'' u ($1*xw-1.5*xw+1):($2*0.9):(gf($3)) w ima, \
'' u ($1*xw-.5*xw+1):($2*0.2):(gf($3)+2.0) w ima, \
'' u ($1*xw+.5*xw+1):($2*0.95):(gf($3)+4.0) w ima, \
'' u ($1*xw-1.5*xw+2):($2*0.95):(gf($3)) w ima, \
'' u ($1*xw-.5*xw+2):($2*0.5):(gf($3)+2.0) w ima, \
'' u ($1*xw+.5*xw+2):($2*1.1):(gf($3)+4.0) w ima, \
'msbar.dat' u ($0-xw):2 w boxes lt -1 lw 0.5 lc rgb "#4f81bd", \
'' u ($0):3 w boxes lt -1 lw 0.5 lc rgb "#4f81bd", \
'' u ($0+xw):4 w boxes lt -1 lw 0.5 lc rgb "#4f81bd"

If you look at the graph, we have three different gradients, and the shadow. That makes four colour schemes altogether. Since we would like to save the difficulties associated with multiplot, we have to cram all those colour schemes into one colour range. More on this a bit later.

So, first we plot the gradient that will fill the bars, and then the "surface" that will represent our shadow. Then we set up our figure: we take off the keys, and unset the colour box. We also specify the width of our bars 'xw', and set the box width accordingly. This latter step is needed, because we want to have a border to the bars. The shadow shift, 'sh' is also defined here. In the next line, we set our colour range, in this particular case, between [0:7]. As we have pointed out, we need 4 colour ranges, and they should not overlap, therefore, we need 3 gaps between these ranges. For the sake of simplicity, we define the gap to be 1, and all the ranges to be one. This is why we end up with a total colour range of [0:7]. If you have more, or less bars to plot, you can re-define this range accordingly. The next couple of steps are trivial, up to the definition of the colour schemes. We want to have only simply gradients, thus, it is enough to define the colours at the end points. If you are unhappy with the colouring of your bars, you should change the colours here.

Having set up the figure, we can plot the data. First, we plot the labels for the xtics, and the values of the bars. Next comes the shadow. It might be a bit of an overkill for this figure, so you can skip all lines up to 'msbar_bar.dat'. Note that we simply go through all the data points in our file, and shift the shadow in each step. The height of the shadow is given by the product of the second column (which takes values between 0 and 1) of 'msbar_bar_sh.dat' and the particular data point in 'msbar.dat'. We also add a small downwards and rightwards shift to the shadows, lest they should be covered by the bars. The most important point, however, is that we plot the shadow as
'' u ($1*xw-.5*xw+sh):($2*1.0-sh):($3+6.0) w ima

i.e., the third column is shifted by 6. We do this, so that the shadow is pushed into the [6:7] range, where the shadow's colour scheme is defined. Plotting the bars happens in a similar fashion, the only difference is that we do not add 'sh' to the columns, and we push the values into the range of the appropriate colour range. At the very end, we plot the data file once more, this time with boxes, so that the bars can have a border to them.

If you want to use this plot many times, it might be worthwhile to implement it in a script in the language of your choice. The only thing required is printing lines and numbers. In pseudocode, it would look something like this:
for i
  for j
     d = read(datafile(i,j))
     print "'msbar_bar_sh.dat' u ($1*xw-(%d-1.5)*xw+sh):($2*%d-sh):($3+6.0) w ima, \", i, d
     print "'msbar_bar.dat' u ($1*xw-(%d-1.5)*xw):($2*%d):($3+%d) w ima, \", i, d, i

Now, suppose that you want to add a legend to the figure. The usual way, setting the key, will not work here, for obvious reasons. However, we can easily add that to the figure. We just have got to find some space for it. Since there is no space left on the figure, we have to make some: we will use multiplot, and specify the size of the main figure to be less, than 1. Therefore, adding the following lines to our gnu script should produce the legend.
set multiplot
set size 0.8, 1
... (The main plot, identical to our original plot)
set origin 0.75, 0
unset border; unset xtics; unset ytics; unset ylabel; unset xlabel
set label 1 at first -0.15, 1.35 "Flatland"
set label 2 at first -0.15, 1.15 "Curvedland"
set label 3 at first -0.15, 0.95 "Land"
plot 'msbar_bar.dat' u ($1*xw-0.4):($2/10.0+1.3):(gf($3)) w ima, \
'msbar_bar.dat' u ($1*xw-0.4):($2/10.0+1.1):(gf($3)+2) w ima, \
'msbar_bar.dat' u ($1*xw-0.4):($2/10.0+0.9):(gf($3)+4) w ima

unset multiplot

The script above results in the figure below:

Friday, 2 October 2009

The turning of the histogram

I have been off for some time now, and I thought that it would be high time to post something on gnuplot. I have chosen an easy subject, but one that could turn out to be useful. At least, I have seen people searching for a solution to this problem with the histograms. If you needed it in the past, you have probably realised that gnuplot can make only vertical histograms. But sometimes, this is not the best way to represent data, and a horizontal version would be much better. For one thing, the horizontal one might take up much less space. If you want to learn how to produce the figure below, keep reading!

Making the graphs will require some work, but everything is quite straightforward. What we should note, however, is that we will still produce an upright image that we have to rotate later. If you set the terminal to postscript, you will probably use the figure in LaTeX, where you simply have to tell the compiler to rotate the figure by 90 degrees. If you set the terminal to a raster format, or print the screen, you can rotate the image in many applications, but if you want to adhere to the command line, you can, at least, under Linux, use the convert command as
convert -rotate 90 figure_in.png figure_out.png

The figure above was produced based on the following data file
1989 0.1
1990 0.2
1991 0.2
1992 0.05
1993 0.15
1994 0.3
1995 0.1

(We have seen these data many times, you should know it by heart by now!) After this interlude, let us see the code! I will discuss it afterwards.
set key at graph 0.24, 0.85 horizontal samplen 0.1
set style data histogram
set style histogram cluster gap 1
set style fill solid border -1
set boxwidth 0.8
set xtic rotate by 90 scale 0
unset ytics
set y2tics rotate by 90
set yrange [0:0.35]; set xrange [-0.5:6.5]
set y2label 'Output' offset -2.5
set xlabel ' '
set size 0.6, 1
set label 1 'Year' at graph 0.5, -0.1 centre rotate by 180
set label 2 'Nowhere' at graph 0.09, 0.85 left rotate by 90
set label 3 'Everywhere' at graph 0.2, 0.85 left rotate by 90
p 'pie.dat' u 2 title ' ', '' u ($2/2.0+rand(0)/10.0) title ' ', '' u 0:(0):xticlabel(1) w l title ''

The first line after the reset is required, because we have to make our key ourselves. We simply specify the coordinates and that that we want to have a horizontal key (i.e., one in which the keys are placed to the right of the previous one), with a length of 0.1. If you want smaller or larger space between the keys, you can modify this, by adding the spacing flag to the set command. You can see the exact syntax by issuing ?key in the gnuplot prompt.
In the next four lines, we set up our histogram. The content of these lines might depend on what exactly we intend to plot. For more on this, check out the gnuplot demo page! We then rotate the xtics by 90 degrees. We do this, because the whole image will be rotated, and by rotating the xtics, we make sure that those will be horizontal at the end. For the very same reason, we also unset the ytics, and set the y2tics. We also set the y2label and xlabel. This latter one is empty, but we still need the space for it, so we set it to ' '. Beware the white space!

The next important step is the setting of the aspect ratio of our figure, which will be 0.6:1. Having done that, we introduce 3 labels: one for the xlabel, and two for the key. The placement of these labels is somewhat arbitrary, and depends on the particular terminal that you use. But only these three numbers and the coordinates of the key need any tweaking, really. At the very end, we plot our data. I plotted the second column twice, with an added random numbers, so that we can have two columns at each data point. Note that we plot the first column, too, but pass it to the xlabel command. By doing that, we can automate the labelling of the xtics, using the first column in our data file. Also note the specification of the titles: in the first two cases, the single quotes include a white space, while in the third one, there is nothing.