Draw box around tick label in Matplotlib

Question

I have a Matplotlib figure with multiple subplots:

I want to draw a small red box around one of the ticklabels, as so:

How can I do this (reliably and repeatably) in Matplotlib?

Answer 1

Easiest Case: A Static Plot

For a static plot, it's straightforward:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.axis([0, 2000, 0, 2e-9])

label = ax.xaxis.get_ticklabels()[-1]
label.set_bbox(dict(facecolor='none', edgecolor='red'))

plt.show()

(Note that you can configure the box around the label in a number of ways if you want to change padding, rounding, shape, etc. The annotation guide has some examples. Look at the bbox and boxstyle examples.)

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Keeping up with interactive changes

However, if we zoom or pan interactively, the tick label with the red border will not necessarily be at 2000. (Instead, it will be index based.)

Panning:

Zooming:

To do it fully repeatably so that it will stay there regardless of how you interactively zoom and pad, you'd need to connect a callback to the draw event.

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Using Annotation Instead of Ticklabels

However, there's an easier way that may suit your purposes better, regardless.

Rather than make it a tick label, draw it using annotation instead. That way you'll always have a label at the specified value, regardless of how the ticks are drawn.

As a very hackish example (normally, you'd probably put a textual label a bit below instead...):

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.axis([0, 2000, 0, 2e-9])

ax.annotate('_____', xy=(2000, 0), xytext=(0, -ax.xaxis.labelpad),
            xycoords=('data', 'axes fraction'), textcoords='offset points',
            ha='center', va='top',
            bbox=dict(boxstyle='round', fc='none', ec='red'))

plt.show()

And no matter how we zoom or pan, it will stay at 2000 on the x-axis (though it doesn't guarantee there will be a tick or tick label at 2000):

More often, though, you'd use this to place or annotate some specific value. For example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.axis([0, 2000, 0, 2e-9])

ax.annotate('Cutoff', xy=(2000, 0), xytext=(0, -15 - ax.xaxis.labelpad),
            xycoords=('data', 'axes fraction'), textcoords='offset points',
            ha='center', va='top',
            bbox=dict(boxstyle='round', fc='none', ec='red'))

plt.show()

Note that this will also stay at the a position of 2000 on the x-axis no matter how we zoom or pan and will be there regardless of whether or not the ticklabel is there.