如何在 matplotlib 上获得科学计数法的乘数字符串

Question

I'm trying to get the text in the offset of the scientific notation of matplotlib, but get_offset() or get_offset_text() returns an empty string. I have checked these questions, but they didn't work:

How to move the y axis scale factor to the position next to the y axis label?

Adjust exponent text after setting scientific limits on matplotlib axis

prevent scientific notation in matplotlib.pyplot

Here is a simple example:

import matplotlib.pyplot as plt
from matplotlib.ticker import ScalarFormatter
import numpy as np

x = np.arange(1,20)
y = np.exp(x)

fig,ax = plt.subplots(1,1)

ax.plot(x,y)

ax.yaxis.set_major_formatter(ScalarFormatter(useMathText=True)) 

print(ax.yaxis.get_offset_text())
print(ax.yaxis.get_major_formatter().get_offset())

fmt = ax.yaxis.get_major_formatter()
offset = ax.yaxis.get_major_formatter().get_offset()
print(offset)


plt.show()

That generates:

I'd like to get the x10^8 , but it returns only:

Text(0, 0.5, '')

The same happens if I don't use the ScalarFormatter. Am I missing something? Is there a separate function to get the multiplier (instead of the offset)?

edit: I'm currently using Python 3.9.0 with matplotlib 3.4.2 on a MacBook Pro, and I just run python3 test.py .

edit2: I have installed Python 3.9.5 , and the solution with fig.canvas.draw() still does not work. The same with Linux works.

edit3: The problem happens when using the MacOSX backend. When changing to TkAgg or Agg , the get_offset works with the provided solution.

Answer 1

You first need to draw the figure for the object to not hold some default values. From the source code on FigureCanvasBase.draw :

"""
Render the `.Figure`.
It is important that this method actually walk the artist tree
even if not output is produced because this will trigger
deferred work (like computing limits auto-limits and tick
values) that users may want access to before saving to disk.
"""

Simply call fig.canvas.draw() and then ax.yaxis.get_offset_text() will have the updated values you want.

x = np.arange(1, 20)
y = np.exp(x)

fig, ax = plt.subplots(1, 1)
ax.plot(x, y)
ax.yaxis.set_major_formatter(ScalarFormatter(useMathText=True))

fig.canvas.draw()
         
offset = ax.yaxis.get_major_formatter().get_offset()
print(offset)
# $\times\mathdefault{10^{8}}\mathdefault{}$