Feat/banded legend values

[nickofthyme]

Summary

Continuation of #391 (first commit comes from #391)

closes #163

Add duplicated values on the legend for banded AREA series. Legend will show upper and lower series values.

Update last values to find last values for y0 and y1

Also "fixes"/removes the tooltip from showing undefined series y0 values as 0. Now undefined y0 values are filtered out of the tooltip. These values will show as blanks in the legend.

Screenshots

Checklist

• Any consumer-facing exports were added to src/index.ts (and stories only import from ../src except for test data & storybook)
• This was checked for cross-browser compatibility, including a check against IE11
• Proper documentation or storybook story was added for features that require explanation or tutorials
• Unit tests were updated or added to match the most common scenarios
• Each commit follows the convention

[codecov-io]

Codecov Report

Merging #398 into master will decrease coverage by 0.05%.
The diff coverage is 92.1%.

@@           Coverage Diff            @@
##           master   #398      +/-   ##
========================================
- Coverage   98.05%    98%   -0.06%     
========================================
  Files          39     39              
  Lines        2833   2853      +20     
  Branches      673    664       -9     
========================================
+ Hits         2778   2796      +18     
  Misses         50     50              
- Partials        5      7       +2

Impacted Files	Coverage Δ
src/chart_types/xy_chart/utils/specs.ts	100% <ø> (ø)	⬆️
...chart_types/xy_chart/utils/stacked_series_utils.ts	98.21% <100%> (ø)	⬆️
src/chart_types/xy_chart/store/utils.ts	96.76% <100%> (+0.05%)	⬆️
src/chart_types/xy_chart/utils/series.ts	100% <100%> (ø)	⬆️
src/chart_types/xy_chart/legend/legend.ts	95% <88.23%> (-5%)	⬇️
src/chart_types/xy_chart/tooltip/tooltip.ts	93.75% <89.47%> (+0.52%)	⬆️

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[markov00]

There are some small comments to solve before merge. Tested locally and I cannot find issues on that