[Sunburst] Single value sometimes shows and 2 slices

[nickofthyme]

When defining a Partition chart, specifically radial types (i.e. sunburst), the chart can incorrectly show a single (100%) slice as 2 slices indicated by the separating line at the top of the pie chart.

The issues here is with this part of the code...

elastic-charts/packages/charts/src/chart_types/partition_chart/layout/viewmodel/viewmodel.ts

Lines 246 to 249 in 27c04f4

	case PartitionLayout.sunburst:
	const sunburstValueToAreaScale = TAU / totalValue;
	const sunburstAreaAccessor = (e: ArrayEntry) => sunburstValueToAreaScale * mapEntryValue(e);
	return sunburst(tree, sunburstAreaAccessor, { x0: 0, y0: -1 }, clockwiseSectors, specialFirstInnermostSector);

Specifically Line 247 which sets the ratio of the total combined value. In some cases, the totalValue can be just right to offset the precision of this value by one decimal, like the case with 678.

TAU = 2 * Math.PI ~= 6.283185307179586
TAU / 678 ~= 0.009267234966341572
0.009267234966341572 * 678 ~= 6.283185307179585 != TAU

This comes into play when we render the sections and check if the x1, or section angle, is equal to TAU or a full circle. With this lose in precision this condition in not met causing the inconsistent rendering of the section divider line.

elastic-charts/packages/charts/src/chart_types/partition_chart/renderer/canvas/canvas_renderers.ts

Line 118 in 27c04f4

if (strokeWidth > 0.001 && !(x0 === 0 && x1 === TAU)) {

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Related to elastic/kibana#184386