Exporting statistics graph as PNG is missing legend and parts of the background are transparent

[juliankehne]

Describe the Bug

When exporting the statistics graph as a png.
Some parts of the PNG background are always transparent and the legend of language/color is missing.
See images below.

Steps to Reproduce

1. Go to 'Statistics'
2. Click on 'Export as PNG'
3. Open the PNG in an image viewer/editor
4. See transparent parts in png and missing legend

Expected Behavior

Complete graph

Actual Behavior

Missing legend of available languages and transparent background

Additional Information

Now:

Before:

Previously the legend was also missing sometimes, not sure what causes it.

Traceback

Related Issues

#3221