`chart.load` should return the assigned color.

It is complex, but not too hard, to predict which color is used when loading data in to a chart. This is especially true when lines can be removed, and colors re-used.

If elements in the page are color-coded, this involves keeping an external available-color registry.

This would be made easier if the `load` function simply returned the color, and any other relevant values, rather the the current result of `undefined`.