Add indexingMode to Chart
Right now, certain items make assumptions about how the colorSource/nameSource relation to valueSources is regarding indexing. PieChart assumes each color/name describes a single value, whereas Bar and Line charts assume each name/color is a single valueSource. For LegendModel we then need to figure this out with extra properties.
Instead of all this, Chart should have a single property indexingMode with the values EachValue, Continuous or EachSource, with different charts specifying different defaults. This would allow switching PieChart to use a mode where colors are based on each value source and BarChart to switch to using a color for each value.