[export] Repair `ggplot` graph rendering by switching to `plotnine`

[amotl]

About

On the HTTP export API, repair ggplot graph rendering.

References

• GH-97
• https://community.hiveeyes.org/t/imkerliche-daten-und-umweltdaten-exportieren/1662/26

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/daq-tools/kotori/pull/147/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/daq-tools/kotori/pull/147/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools) | `81.21% <100.00%> (+2.61%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/daq-tools/kotori/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools) | Coverage Δ | | |---|---|---| | [kotori/io/export/plot.py](https://app.codecov.io/gh/daq-tools/kotori/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools#diff-a290b3JpL2lvL2V4cG9ydC9wbG90LnB5) | `53.90% <100.00%> (+40.62%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/daq-tools/kotori/pull/147/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daq-tools)

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