如何在 Gnuplot 中以编程方式使用轴？

Question

If you need to draw some graphs against x1y1 or x1y2 axes, depending on the maximum y value of each graph, what would be the proper syntax for that?

My data file has several columns. The first one contains x values, the others y1, y2 and so on. After using the stats command, I could easily define each axis value for each graph:

stats '$data_file' u 1:2 nooutput;
y1max = STATS_max_y;
if (y1max > ymax) { y1axis = 'x1y2' } else { y1axis = 'x1y1' };
stats '$data_file' u 1:3 nooutput;
y2max = STATS_max_y;
if (y2max > ymax) { y2axis = 'x1y2' } else { y2axis = 'x1y1' };
...

After that, I can draw the graphs with this plot cmd

plot '$data_file' using 1:2 axes x1y1 notitle with lines lc rgb 'black' lw 1,\
               '' using 1:3 axes x1y2 notitle with lines lc rgb 'green' lw 1;

which works, but it is not programmatically at all.

But this one doesn't works

plot '$data_file' using 1:2 axes @y1axis notitle with lines lc rgb 'black' lw 1,\
               '' using 1:3 axes @y2axis notitle with lines lc rgb 'green' lw 1;

where I used the Substitution of string variables as macros (The character @ is used to trigger substitution of the current value of a string variable into the command line...) which is written in the docs .

Nor the eval command likes to work for me.

Could you please provide an example for that purpose, which is working or any a good advise. THX!

Answer 1

I got two errors/warnings when using the above code in a bash script as an oneliner the following way

echo "gnuplot cmds;plot ..." | gnuplot

• warning: y3axis is not a string variable
• axes must be x1y1, x1y2, x2y1 or x2y2

As written in the docs, the macro could not be expanded the same time as it is defined , what is the case if written on the same line.

To avoid the above mistake we have to use only

echo -e "gnuplot cmds; \n plot..." | gnuplot plot..." | gnuplot

and it works like it should.

Credits to @Christoph and @Ethan who put me on the right track.