Detect correct level color for gauge with more than one data value per series

[GDFaber]

• Summed up data values using reduce() on the values array of the series, used this sum to detect level color.
• Added tests for changes in arc.js and legend.js.
• This resolves #2750.

[codecov-io]

Codecov Report

Merging #2755 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2755   +/-   ##
======================================
  Coverage    82.9%   82.9%           
======================================
  Files          60      60           
  Lines        4791    4791           
======================================
  Hits         3972    3972           
  Misses        819     819

Impacted Files	Coverage Δ
src/arc.js	75.08% <100%> (ø)	:arrow_up:
src/legend.js	82.97% <100%> (ø)	:arrow_up:

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