A legend of PieChart needs to be laid-out in the following way (should apply also to the case with width/height switched):
* There a free width for the legend, the height of the legend is determined for this free width
* We need the legend to be as tight as possible, so we determine the preferred width based on the height we just computed

PieChart uses TilePane for this, other layout panes should be also capable of such functionality.