[export] Repair ggplot graph rendering by switching to plotnine

[amotl]

About

On the HTTP export API, repair ggplot graph rendering.

References

• Unhandled exception: module 'pandas' has no attribute 'tslib' #97
• https://community.hiveeyes.org/t/imkerliche-daten-und-umweltdaten-exportieren/1662/26

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +2.61% 🎉

Comparison is base (6c32425) 78.59% compared to head (d84acd6) 81.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
+ Coverage   78.59%   81.21%   +2.61%     
==========================================
  Files          55       55              
  Lines        3014     3008       -6     
==========================================
+ Hits         2369     2443      +74     
+ Misses        645      565      -80     

Flag	Coverage Δ
unittests	81.21% <100.00%> (+2.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
kotori/io/export/plot.py	53.90% <100.00%> (+40.62%)	⬆️

... and 2 files with indirect coverage changes

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