Summary

This PR adds the wiggle and silhouette stacks mode as available in vislib.
t is also to fix fit functions for stacked charts #450

Configuring a stackMode in a stacked area or bar chart will change the baseline offset used to render each group of stacks. StackMode are:

export const StackMode = Object.freeze({
  /** Applies a zero baseline and normalizes the values for each point such that the topline is always one. */
  Percentage: 'percentage' as const,
  /** Shifts the baseline so as to minimize the weighted wiggle of layers. */
  Wiggle: 'wiggle' as const,
  /** Shifts the baseline down such that the center of the streamgraph is always at zero. */
  Silhouette: 'silhouette' as const,
});

Applied to a stacked area chart it can be used to stream graphs

close #715

Additional changes

• Fit functions are now working also on stacked charts
  close Fit functions for stacked line and area charts #450
• The legend extra value is now displayed, on-demand, only on charts with a continuous x-axis
  fix Don't display legend extra values when the x scale is ordinal #766
• The PR adjust slightly the way fitted data is represented in the case of null values and no fit option specified. It renders an empty area segment as in the following screenshot

• The PR also adjust how we fit the chart to the data domain: for zero-based charts (bar and area charts) the fit option doesn't have effects, thus the changes on the screenshots.

• The PR finally adjust the clipped ranges for ordinal charts, where the lines where wrongly clipped due to a missing translate call when rendering.

• Some tests are converted to functional tests, instead of unit tests.

BREAKING CHANGE:

• PointStyleAccessor and BarStyleAccessor now calls the method with a DataSeriesDatum as first argument instead of a RawDataSeriesDatum
• stackAsPercentage prop on BarsSpec and AreaSpec is replaced by stackMode that accept one StackMode.

Checklist

• Any consumer-facing exports were added to src/index.ts (and stories only import from ../src except for test data & storybook)
• 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