Fig. 1 Mercator projection of the world (code to produce this figure, data)

In order to achieve the Mercator projection, we apply the following function.

set angles degrees
mercator(latitude) = log( tan(180/4.0 + latitude/2.0) )
set yrange [-3.1:3.1]
plot 'world_110m.txt' u 1:(mercator($2)) w filledcu ls 2

Fig. 2 Equirectangular projection of the world (code to produce this figure, data)

By just plotting the data as we have done for Fig. 2, we have the Equirectangular projection with constant spacing between the latitudes and meridians. The blue background color in the first two figures can be achieved directly with a terminal setting.

set terminal pngcairo size background '#c8ebff'

Fig. 3 Mapping of the Mercator projection (code to produce this figure)

In Fig. 3 the Mercator projection function is shown as an input-output-function of the latitude values. The placing of the latitude values on the y-axis can be easily done with a loop.

set ytics 0
do for [angle=-80:80:20] {
    set ytics add (sprintf('%.0f',angle) mercator(angle))
}

Fig. 1 An animated spiral trajectory (code to produce this figure, data)

In order to create the animation we have to produce a set of png images and create the resulting gif animation with GIMP as shown in the Animation I – gif entry. Therefor, we have to tell gnuplot at which point of the data it has to stop for each image. This can be achieved by the every option. The point at the end of the line is just one data point. Here the start point and end point for every are just the same.

do for [ii=1:99] {
    splot 'spiral.txt' every ::1::ii w l ls 1, \
          'spiral.txt' every ::ii::ii w p ls 1
}

The downward spiral is created by running the loop in the other direction.

do for [ii=99:1:-1] {
    splot 'spiral.txt' every ::1::ii w l ls 1, \
          'spiral.txt' every ::ii::ii w p ls 1
}

By the way, I don’t know why the antialiasing of the output png images is not working in this example. If you have any idea, feel free to tell me.Assume you have a data file describing a trajectory that you would like to animate like the spiral shown in Fig. 1.

In comparison to the additional file for the loop in Animation I – gif, now all we need is this small code block.

do for [ii=1:181] {
    set output sprintf('hrir_frame%03.0f.png',ii)
    set multiplot layout 2,1
    [...]
    plot 'ir.txt' u ($1*1000):2*ii-1 w l ls 1
    [...]
    plot 'ir.txt' u ($1*1000):2*ii w l ls 1
    [...]
}

Until 4.6 an iteration over different lines of code was only possible with the help of an extra file. This technique was used, for example, for the gif animation entry. There the loop was executed by

t = 0
end_time = 1
load 'bessel.plt'

with the file bessel.plt containing the code to execute within the loop

# bessel.plt
t = t + 0.02
outfile = sprintf('animation/bessel%03.0f.png',50*t)
set output outfile
splot u*sin(v),u*cos(v),bessel(u,t) w pm3d ls 1
if(t<end_time) reread;

This can now be reformulated in a much shorter way by applying the new do command and the block-structure given by the { }

do for [t=0:50] {
    outfile = sprintf('animation/bessel%03.0f.png',t)
    set output outfile
    splot u*sin(v),u*cos(v),bessel(u,t/50.0) w pm3d ls 1
}

Now there is no need for an additional file. The only thing to consider is the change of the index t, because for the for-loop t has to be an integer.

The block-structure can in the same way be applied to the if-statement.