Fix #140: Add ability to disable automatic showSelected on legends

[ANAMASGARD]

Issue

FIXES #140

Problem

Using aes(color=variable) forces showSelected behavior on the legend with no opt-out.

This PR { First commit }

Adds failing test proposing showSelected=character() to disable automatic selector.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (68b074e) to head (8dcae4f).

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #292       +/-   ##
===========================================
+ Coverage   73.06%   95.46%   +22.39%     
===========================================
  Files         164        1      -163     
  Lines        8758     2754     -6004     
  Branches        0      554      +554     
===========================================
- Hits         6399     2629     -3770     
+ Misses       2359      125     -2234     

Flag	Coverage Δ
javascript	95.46% <ø> (+14.70%)	⬆️
r	?

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[tdhock]

• HEAD=fix/legend-without-showSelected slower P<0.01 for getCommonChunk improved in #238
  

Generated via commit 8dcae4f

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 32 seconds
Installing different package versions	59 seconds
Running and plotting the test cases	3 minutes and 46 seconds

[ANAMASGARD]

Regarding performance

• The slight slowdown is due to the additional check for showSelected = character().
• This check is O(1) and runs once per legend per layer.
• Happy to optimize if needed, but the impact { slowdown } seems minimal.
• The ghpages test failure is unrelated , All 2116 other tests pass, including the 8 new legend tests.