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

[Glenn Nelson]

I believe I saw an excellent example of this using Bokeh. There was a map,
and you could drag a rectangle in it and have that area show up in a linked
box. Also could move the rectangle around. Sorry for lack of information -
it's on my work computer and I'm not there. I realize this does not use
matplotlib, but it's good to know about alternative ways of doing things.

----
Glenn Nelson in Santa Cruz
social: http://google.com/+GlennNelson
see my Kite Aerial Photos at http://www.glenn-nelson.us/kap

On Thu, Mar 8, 2018 at 7:51 AM, Chad Parker <parker.charles at gmail.com>
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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