add multiple axis functionality

[slaclau]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 18.16%. Comparing base (156deed) to head (ecabaa7).

Files with missing lines	Patch %	Lines
src/plotly_gtk/demo.py	0.00%	18 Missing ⚠️
src/plotly_gtk/chart.py	0.00%	6 Missing ⚠️
src/plotly_gtk/_chart.py	0.00%	4 Missing ⚠️
src/plotly_gtk/widgets/axis_title.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   18.54%   18.16%   -0.38%     
==========================================
  Files          14       14              
  Lines        1197     1222      +25     
==========================================
  Hits          222      222              
- Misses        975     1000      +25     

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