fix(xy): disable legend extra on ordinal (#2114)

This commit disables the legend extra value when using an ordinal scale.
The major changes, visible also on the changes in the screenshot testing are
- no space left at the right of each legend item for ordinal x scale series
- no visible current value in the legend item when hovering over a chart with ordinal values

BREAKING CHANGE: when using the `ScaleType.Ordinal` for the X scale the legend extra value, representing the last and current hovered value, will not be shown.

packages/charts/src/chart_types/xy_chart/state/selectors/get_legend_item_extra_values.ts

@@ -6,14 +6,19 @@
  * Side Public License, v 1.
  */
 
+import { getComputedScalesSelector } from './get_computed_scales';
 import { getTooltipInfoAndGeomsSelector } from './get_tooltip_values_highlighted_geoms';
 import { LegendItemExtraValues } from '../../../../common/legend';
 import { SeriesKey } from '../../../../common/series_id';
+import { ScaleType } from '../../../../scales/constants';
 import { createCustomCachedSelector } from '../../../../state/create_selector';
 import { getLegendItemExtraValues } from '../../tooltip/tooltip';
 
+const EMPTY_MAP = new Map();
 /** @internal */
 export const getLegendItemExtraValuesSelector = createCustomCachedSelector(
-  [getTooltipInfoAndGeomsSelector],
-  ({ tooltip: { values } }): Map<SeriesKey, LegendItemExtraValues> => getLegendItemExtraValues(values),
+  [getTooltipInfoAndGeomsSelector, getComputedScalesSelector],
+  ({ tooltip: { values } }, { xScale: { type } }): Map<SeriesKey, LegendItemExtraValues> =>
+    // See https://github.com/elastic/elastic-charts/issues/2050
+    type === ScaleType.Ordinal ? EMPTY_MAP : getLegendItemExtraValues(values),
 );

packages/charts/src/chart_types/xy_chart/state/utils/get_last_value.ts

@@ -8,6 +8,7 @@
 
 import { LastValues } from './types';
 import { SeriesKey } from '../../../../common/series_id';
+import { ScaleType } from '../../../../scales/constants';
 import { XDomain } from '../../domains/types';
 import { isDatumFilled } from '../../rendering/utils';
 import { DataSeries, getSeriesKey, XYChartSeriesIdentifier } from '../../utils/series';
@@ -19,6 +20,10 @@ import { StackMode } from '../../utils/specs';
  * @param xDomain
  */
 export function getLastValues(dataSeries: DataSeries[], xDomain: XDomain): Map<SeriesKey, LastValues> {
+  // See https://github.com/elastic/elastic-charts/issues/2050
+  if (xDomain.type === ScaleType.Ordinal) {
+    return new Map();
+  }
   const lastValues = new Map<SeriesKey, LastValues>();
 
   // we need to get the latest