matplotlib freeze with `savefig()`

[thomashampson]

Describe the bug
MantidWorkbench will hang when the script below is run.

To Reproduce
Run the following script inside the script editor:

# import mantid algorithms, numpy and matplotlib
from mantid.simpleapi import *
import matplotlib.pyplot as plt
 
a = [1,2,3,4]
b = [5,6,7,9]
 
ws = CreateWorkspace(a,b)
 
fig, axes = plt.subplots(subplot_kw={'projection': 'mantid'})
axes.plot(ws, color='#1f77b4', label='label', marker='o', markersize=3.0, wkspIndex=0)
axes.set_title('test_data')
axes.set_xlabel('y-scale')
axes.set_ylabel('x-scale')
axes.set_xlim([0, 5])
axes.set_ylim([0, 10])
fig.show()
fig.savefig('plot1.png')

Platform/Version (please complete the following information):

• OS: Windows confirmed, not tested anything else
• Mantid Version: 6.13.1, 6.14.0, maybe earlier

Additional context