KeyError bugfix for matplotlib bar chart

[dorisjlee]

This PR fixes the issue in #380. In addition, this PR includes several fixes to matplotlib code exports.

[codecov]

Codecov Report

Merging #391 (dfabe0b) into master (29e0c70) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
+ Coverage   84.91%   84.93%   +0.02%     
==========================================
  Files          52       52              
  Lines        4024     4030       +6     
==========================================
+ Hits         3417     3423       +6     
  Misses        607      607              

Impacted Files	Coverage Δ
lux/vislib/matplotlib/BarChart.py	92.59% <100.00%> (+0.38%)	⬆️
lux/vislib/matplotlib/LineChart.py	88.73% <100.00%> (+0.16%)	⬆️
lux/vislib/matplotlib/ScatterChart.py	76.74% <100.00%> (+0.27%)	⬆️

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