#340 Align origin of axes y1 and y2 (or x1 and x2)

[Anonymous]

When plotting data on 2 different axes, the pointis often to compare relative variations of data in different units. Thus it is often more correct to plot them with the same position of the zero, so that relative variations are to scale.

In gnuplot, this is easily done for positive data, setting ymin=y2min=0, but I can find no way to do it with autoscale for data that can change sign.

[Anonymous]

I can do it manually of course :
[code]
plot "data2.dat" axes x1y2
SAVE_Y2_MIN=GPVAL_Y2_MIN
SAVE_Y2_MAX=GPVAL_Y2_MAX
plot "data1.dat" axes x1y2
max(a,b)=(a>b)?a:b
a=max(SAVE_Y2_MIN/GPVAL_Y_MIN,SAVE_Y2_MAX/GPVAL_Y_MAX)
set y2range [GPVAL_Y_MIN*a:GPVAL_Y_MAX*a]
replot "data2.dat" axes x1y2
[/code]

[Ethan Merritt]

Version 4.7 now supports a command "set link {x2|y2}" that I think will do what you want. For example, if you wanted y axis data in Farenheit and y2 axis data in Centigrade, but aligned so that 32F is parallel to 0C

c2f(y) = 32. + 9.*y/5.
f2c(y) = (y-32.) * 5./9.
set link y2 via f2c(y) inverse c2f(y)