set_ticks() changes view limits of the axis

Bug report

Bug summary

It seems that calling ax.set_ticks() after the axis range has been defined with ax.set_xlim() will change that range invisibly.

This seems to me like a bit of a trap for unwary plotters.

The example code constructs a plot with two axes meant to be the same, but depending on the value of the ticks set they can end up being different.

Code for reproduction

import matplotlib.pyplot as plt

filename = "plot.png"

fig, ax1 = plt.subplots()
ax1.set_xlabel("X")  
ax1.set_ylabel("Y")
ax1.tick_params('y', colors='k')
ax1.set_ylim([0, 1])

ax2 = ax1.twiny()
ax2.set_ylim([0, 1])

ax2.tick_params(axis="x", colors="blue")
ax2.set_xlabel('Test2', color='b')

xx =[4.2, 4.3, 5.0]    
#xx =[4.2, 4.3]    # Try uncommenting this line!

ax1.set_xlim([4.0, 4.5])
ax2.set_xlim([4.0, 4.5])

## This changes xmin, xmax depending on values of xx 
## and makes ax2 different to ax1
ax2.xaxis.set_ticks(xx) 


plt.savefig(filename)

Actual outcome

plot

Expected outcome

I would expect the plot to show the two axes (blue and black X axis) agreeing.
Note if I edit the above code to move the set_ticks() call before the set_xlim() call this does happen. But I would not expect the order these are called to matter.

A quick look at the set_ticks() source code shows it auto-expands the axis range!
https://matplotlib.org/3.1.3/_modules/matplotlib/axis.html#Axis.set_ticks

I am not sure of the logic behind this, perhaps it should be reconsidered?

This might be same as issue #8320 ?

Matplotlib version

  • Operating system: Mac OS 10.14.6
  • Matplotlib version: 3.1.3
  • Matplotlib backend (print(matplotlib.get_backend())):
  • Python version: 3.7.6

Used anaconda

1 possible answer(s) on “set_ticks() changes view limits of the axis

  1. I’m not sure this is something that can be changed. I think the best thing here is to update the docstring of set_ticks() to mention that the axis limits are also updated by the method.