Matplotlib export: Fix code for different shapes and sizes

[janezd]

Issue

Fixes biolab/orange3#7258.

Description of changes

• Different sizes failed because of sizes == -1 where sizes could be an array. np.all works is safer.
• In fixing incorrect code for different shapes I also replace np.where with the preferred np.nonzero (though result is the same).

Includes

• Code changes
• Tests

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.36%. Comparing base (cbf5b10) to head (ce2da6c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   72.07%   72.36%   +0.28%     
==========================================
  Files          34       34              
  Lines        9126     9126              
==========================================
+ Hits         6578     6604      +26     
+ Misses       2548     2522      -26     

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