Pie chart labels at start or end of arc instead of center

I'd like another parameter other than `align` and `anchor` to be used for label placement on pie/doughnut charts along the arc. I'm not sure what to call it, but `arcPos` might be good.

I'd like it to be a value between 0.0 and 1.0 where 0.0 is the beginning of the arc, and 1.0 is the end of the arc. The label placement for pie and doughnut charts currently is stuck at an `arcPos` of 0.5.

Even if you wanted to stick to the scheme of `"begin"`, `"center"`, or `"end"`, I'd be fine with that, but having a value of 0.0 to 1.0 is more flexible.