[Plot] Fix plot swatch behavior when vertical space is small

[rukmini-bose]

Closes #7490

Describe your changes:

• Add overflow: hidden to swatch container such that when the plot is vertically compressed, the container does not scroll, but rather hides overflow swatches.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.17%. Comparing base (ceeb761) to head (a3a81ba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7493      +/-   ##
==========================================
+ Coverage   56.15%   56.17%   +0.01%     
==========================================
  Files         672      672              
  Lines       27112    27112              
  Branches     2635     2635              
==========================================
+ Hits        15226    15230       +4     
+ Misses      11561    11557       -4     
  Partials      325      325              

Flag	Coverage Δ
e2e-full	23.60% <ø> (ø)
e2e-stable	59.96% <ø> (+0.02%)	⬆️
unit	49.09% <ø> (-0.03%)	⬇️

see 2 files with indirect coverage changes

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[charlesh88]

LGTM!

[rukmini-bose]

Testing Instructions:

1. Make an Overlay Plot with maybe 4+ items in at least one of the Y-Axis.
2. Drag that into a Display Layout.
3. Make the Overlay Plot vertically very small, then expand the legend.
4. Verify that the overflow swatches are being hidden under the legend, such that the legend is visible.