Allow Y axis on RHS of SVG plot

[hyanwong]

This is a bit trivial, but it was the easiest way to make nice tanglegram plots for sc2ts, in which trees are rotated in different directions (see below). So I thought as I had done the work, I might as well see if it is sensible to include in tskit? Otherwise I'll just keep it on my own branch.

Note that this allows y_axis=True or y_axis="right", which is a bit yucky. No tests either, in case it seems a silly idea:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.63%. Comparing base (e81ee40) to head (d59218e).
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##             main    #3201   +/-   ##
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  Coverage   89.63%   89.63%           
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  Files          28       28           
  Lines       31978    32000   +22     
  Branches     5872     5877    +5     
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+ Hits        28662    28684   +22     
  Misses       1886     1886           
  Partials     1430     1430           

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lwt-tests	80.38% <ø> (ø)
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Files with missing lines	Coverage Δ
python/tskit/drawing.py	98.39% <100.00%> (+0.02%)	⬆️
python/tskit/trees.py	98.88% <ø> (ø)

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[benjeffery]

Seems useful - if you don't like to overloaded arg then adding another arg that is y_axis_right would be possible. I'm ok with the overloaded arg in the "softer" parts of the API like this. I would change the code to only allow the strings "left" and "right" as currently a misspelling gives you odd behavior.

[hyanwong]

I'm happy to overload the argument, and just allow "left" and "right".

[hyanwong]

Should we default to having a Y axis be shown? I think there was an issue about this.

[hyanwong]

OK, added the tests now (hard to test the actual layout, so I added it to two existing svg mockups), e.g.

[jeromekelleher]

LGTM. minor logic tweak suggested