Refine “high cardinality” warning #1668
Merged
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The intent of this “high cardinality” warning (as specified here) is the following: when a mark only¹ makes sense with grouping—currently area, line, linearRegression and density—, we want to warn when the number of groups (G.size) is higher than the smallest possible number of meaningful groups, which is I.length >> 1 (obtained when the groups are pairs).
However when there is an odd number of elements, having 1 element that falls out is to be expected and should not invalidate the chart.
Closes #1667
¹ It's not a hard rule: the areaY mark with a stroke, the lineY mark with a marker, and the density mark tolerate single points. But these are not the default usage.