[Matplotlib-users] Scaled Secondary Axis that is Slaved to Primary

[Juan Nunez-Iglesias]

Hi Chad,

 Tom Caswell answered a similar question from me last year:
https://mail.python.org/pipermail/matplotlib-users/2017-May/000888.html
Look towards the very end of the example to see how to do arbitrary
things automatically when xlims and ylims change.
HTH!

Juan.


On Fri, Mar 9, 2018, at 2:51 AM, Chad Parker wrote:
> All-
> I frequently find myself trying to create plots that use a secondary
> axis to indicate data in a second set of units. For example, I might
> plot a data set with an x-axis in data number (e.g. the output of an
> analog to digital converter), and then wish to display the calibrated
> units on a secondary x-axis (e.g. volts).> 
> There are quite a few examples that do this by creating a secondary
> axis using twiny(), and then  setting the limits of the secondary x-
> axis to the scaled limits of the primary, and possibly setting the
> ticks to line up as well.> import matplotlib.pyplot as plt
> import numpy as np
> from scipy.stats import norm
> 
> f, ax = plt.subplots(1)
> x_dn = np.arange(4096)
> y = norm.pdf(x_dn, 2048, 64)
> v = lambda x: x*5.0/4096
> 
> ax.plot(x_dn, y)
> ax.grid(True)
> ax_top = ax.twiny()
> ax_top.grid(True)
> ax_top.set_xticks([v(x) for x in ax.get_xticks()]) # optional, aligns
> the grids> ax_top.set_xlim([v(x) for x in ax.get_xlim()])
> 
> This isn't too painful if you're only doing it once. However, if you
> subsequently want to change the limits (from the command line) you
> have to explicitly set the limits of both axes or they will become out
> of sync (aside: they also can get out of sync if you set the secondary
> limits before the ticks, because setting the ticks can change the
> limits!). If you do set the ticks to line up the grids, then you also
> have to recompute those for the secondary axis.> ax.set_xlim([1500, 2500])  # now they're out of sync
> ax_top.set_xticks([v(x) for x in ax.get_xticks()])  # ticks correct,
> but in wrong places> ax_top.set_xlim([v(x) for x in ax.get_xlim()])  # all's well again.
> It just seems like there ought to be a better way. I apologize if it's
> out there and I missed it.> Thanks,
> --Chad
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