Add line plots

[connoraird]

Adding the option to plot line charts.

[pineapple-cat]

Looking good so far! Don't forget to update any unit test that has a config with "plot_type": "generic", at the start so we can make sure the CI is passing cleanly.

[pineapple-cat]

This has remained undocumented thus far, but I've been using flake8 with a 120 char/line limit for style enforcement. Perhaps it's time to add a linter to the CI...

Edit: With regards to the current failing checks, pytest passes for me locally and nothing I added should have broken the CI. I think I had a similar issue with Kaan's PR, but can't quite remember how we solved it.

[pineapple-cat]

Good work, very close to being done I think. I've just made a few more notes and noticed that a good final touch to add would be to update the README config structure and config template sections to account for the new custom axis ranges.

[pineapple-cat]

I think I'm more or less happy with the way this PR looks now. The important functionality is all there and I don't see any more major bugs that need to be fixed. Pytest is still passing locally.

Once this gets merged, I think any of the following bits that aren't implemented here should collectively belong to some sort of line plot enhancement issue:

• Update README config structure + template sections to account for new custom axis ranges.
• Implement custom axis ranges for bar charts OR hide range selectors. (For now could also add tooltips to range selectors with this information.)
• Allow use of custom values for only one of min or max.