Fix height scaling issue in TreeSequence.draw_svg.

[jeromekelleher]

Closes #319.

cc @gtsambos

[gtsambos]

Sorry I didn't get around to this quickly - thanks @jeromekelleher!

[jeromekelleher]

The fix works, but tests fail because of another bug. When #383 is fixed, we can rebase this and merge.

[codecov]

Codecov Report

Merging #378 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   86.59%   86.59%           
=======================================
  Files          20       20           
  Lines       14139    14139           
  Branches     2756     2756           
=======================================
  Hits        12244    12244           
  Misses        977      977           
  Partials      918      918

Flag	Coverage Δ
#c_tests	87.71% <ø> (ø)	⬆️
#python_c_tests	90.49% <ø> (ø)	⬆️
#python_tests	99.3% <ø> (?)

Impacted Files	Coverage Δ
python/tskit/drawing.py	99.63% <ø> (ø)	⬆️

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