Tweak use plot legend and significance-drop plot title

[alurban]

This PR tweaks the legend text in auxiliary use plots, and the title position in significance drop plots. Example output is available here (requires LIGO.ORG credentials).

This addresses a request by Sheila Dwyer in ligo-summary-pages.

cc @jrsmith02, @duncanmmacleod

[codecov-io]

Codecov Report

Merging #123 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   53.81%   53.81%           
=======================================
  Files           7        7           
  Lines         788      788           
=======================================
  Hits          424      424           
  Misses        364      364

Flag	Coverage Δ
#Linux	53.81% <100%> (ø)	⬆️
#python27	53.68% <100%> (ø)	⬆️
#python35	53.17% <100%> (ø)	⬆️
#python36	53.17% <100%> (ø)	⬆️
#python37	53.17% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
hveto/plot.py	54.05% <100%> (ø)	⬆️

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[jrsmith02]

Looks good, if you're ok with checks not passed then I can merge.

[alurban]

Thanks for taking a look @jrsmith02. I'm not quite done with this one yet because the formatting of titles on significance-drop plots is still a bit wonky (in particular, the title and subtitle overlap with each other). I'll leave you a message and resolve WIP status once I've got that straightened out.

[alurban]

@jrsmith02, this is now ready for your eyeballs, feel free to merge if you're happy with the changes. I've merged significance plot title and subtitle into one object (a tittle), changed the figure size (of significance drop plots) to 20 x 5, and used bbox_inches='tight' for a tight layout, in addition to the legend label changes that Sheila suggested.

[jrsmith02]

Both plot updates look good to me.