setting tickslabel in matplotlib disables mouse cursor position

Question

Whenever I plot something in matplotlib, moving the mouse over the plot shows at the bottom the x and y position of the cursor. This is very convenient when exploring data.

Now, If I set the ticklabels to something else, the x position of the cursor at the bottom of the plot is empty, while the gui keeps track of the y position. Is there a way to keep getting the x position?

This is a simple example of this happening:

import numpy as np
fig, ax = plt.subplots()
x=np.linspace(0,np.pi)
y=np.sin(x)
ax.plot(x,y)
ax.set_xticks([0,np.pi/2,np.pi])
ax.set_xticklabels(['0','pi/2','pi'])
plt.show()

Interesting. As a side note, if you use ['0',r'$\pi/2$',r'$\pi$'] you can get the labels rendered as TeX. — tacaswell
– tacaswell, Commented Sep 26, 2012 at 16:47

Community · Accepted Answer · 2017-05-23 11:56:17Z

3

No, there is no easy way to do this.

When you set the labels with a list of strings you set the xaxis.major_formatter to be FixedFormatter (doc) which has some peculiar behavior to make it work (it always returns '', unless you give it a third argument which given when it labels the x-axis, but not otherwise). The marking of where the cursor is is generated using the major_formatter and when you call a FixedFormatter it returns '', which is what is displayed.

If you really want this, you have to write your own call back, see Color values in imshow for matplotlib? for a starting point. (or subclass axes and re-implement format_coord (around line 2903 in axes.py in current matplotlib master branch)

edited May 23, 2017 at 11:56

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answered Sep 26, 2012 at 17:21

tacaswell

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Comments

MestreLion · Accepted Answer · 2024-11-01 18:54:26Z

Well, x is not exactly always empty: it will display the label itself if you hover over an exact value corresponding to an assigned label, excluding the extremities. So in your case, hovering on the exact central vertical axis of the figure will yield pi/2:

Not very helpful, I know.

As indicated by tacaswell's answer, you could re-implement ax.format_coord(). It does not actually require you to subclass axes, it can be much simpler than that

In your case, you can simply override ax.fmt_xdata():

import matplotlib.pyplot as plt
import numpy as np
fig, ax = plt.subplots()
x=np.linspace(0,np.pi)
y=np.sin(x)
ax.plot(x,y)
ax.set_xticks([0,np.pi/2,np.pi])
ax.set_xticklabels(['0','pi/2','pi'])
ax.fmt_xdata = lambda x: f"{x:.3f}"  # <-- the magic happens here
plt.show()

You can even go really fancy and format x in terms of pi:

import fractions
import matplotlib.pyplot as plt
import numpy as np
fig, ax = plt.subplots()
x=np.linspace(0, np.pi)
y=np.sin(x)
ax.plot(x,y)
ax.set_xticks([0,np.pi/2,np.pi])
ax.set_xticklabels(['0','pi/2','pi'])
def fmt_xdata(val):
    num, div = fractions.Fraction(val/np.pi).limit_denominator(100).as_integer_ratio()
    num_str = f"{str(num)[:-1] if abs(num) == 1 else num}\u03C0"  # 1pi => pi, -1pi => -pi
    div_str = "" if div == 1 else f"/{div}"  # pi/1 => pi
    return f" {val:.3f} = {val/np.pi:.3f}\u03C0 \u2248 {num_str if num else '0'}{div_str} "
ax.fmt_xdata = fmt_xdata
plt.show()

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