make it possible to indicate multiple x and y facets in mode 2

[leeoniya]

for scale ranging, 2d objects like heatmap cells can have xMin xMax yMin yMax facets that should be used for finding scale range. currently only a single x and y facet is used, leading to clipping of data.

uPlot/src/uPlot.js

Lines 1323 to 1330 in 38f63de

	// TODO: only handles, assumes and requires facets[0] / 'x' scale, and facets[1] / 'y' scale
	let [ xFacet, yFacet ] = s.facets;
	let xScaleKey = xFacet.scale;
	let yScaleKey = yFacet.scale;
	let [ xData, yData ] = data[i];
	accScale(wipScales[xScaleKey], pendScales[xScaleKey], xFacet, xData, xFacet.sorted);
	accScale(wipScales[yScaleKey], pendScales[yScaleKey], yFacet, yData, yFacet.sorted);