Plots y axis and legend fixes

[shefalijoshi]

Closes #5058, #5057

Describe your changes:

Set the unit for a plot series if the yKey changes after initialization
Show y axis label if all series in the plot have the same range value.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Is this change backwards compatible? For example, developers won't need to change how they are calling the API or how they've extended core plugins such as Tables or Plots.

Author Checklist

• Changes address original issue?
• Unit tests included and/or updated with changes?
• Command line build passes?
• Has this been smoke tested?
• Testing instructions included in associated issue?

Reviewer Checklist

• Changes appear to address issue?
• Changes appear not to be breaking changes?
• Appropriate unit tests included?
• Code style and in-line documentation are appropriate?
• Commit messages meet standards?
• Has associated issue been labelled unverified? (only applicable if this PR closes the issue)
• Has associated issue been labelled bug? (only applicable if this PR is for a bug fix)

[codecov]

Codecov Report

Merging #5062 (2705a7a) into master (21ae9f4) will increase coverage by 0.00%.
The diff coverage is 55.55%.

@@           Coverage Diff           @@
##           master    #5062   +/-   ##
=======================================
  Coverage   49.89%   49.89%           
=======================================
  Files         525      525           
  Lines       19218    19226    +8     
  Branches     1733     1734    +1     
=======================================
+ Hits         9588     9593    +5     
- Misses       9203     9206    +3     
  Partials      427      427           

Impacted Files	Coverage Δ
src/plugins/plot/axis/YAxis.vue	66.66% <0.00%> (-6.07%)	⬇️
src/plugins/plot/MctPlot.vue	52.30% <66.66%> (+0.38%)	⬆️
src/plugins/plot/configuration/PlotSeries.js	67.07% <100.00%> (+0.20%)	⬆️
src/api/objects/ObjectAPI.js	91.16% <0.00%> (-2.21%)	⬇️
src/ui/router/Browse.js	86.48% <0.00%> (-1.36%)	⬇️
src/ui/inspector/details/Properties.vue	62.50% <0.00%> (+1.78%)	⬆️
src/api/composition/DefaultCompositionProvider.js	81.81% <0.00%> (+4.54%)	⬆️

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[davetsay]

Great (and quick) work! LGTM!