For your case, there are two methods:
(a) Put the XYCharts into the MultiChart, and leave the PieChart as a separate chart. You can use two ChartViewers on the two charts so the web page can display all the charts you need.
(b) Merge the PieChart into the XYChart, as that it becomes part of the XYChart. The PieChart is then just like a large logo image or background image in the XYChart. You just need to create an XYChart will some empty space reserved for the PieChart. The merge code is something like:
XYChart c = new XYChart(....);
... set up the complete XYChart as usual ...
PieChart p = new PieChart(....);
... set up the complete PieChart as usual ...
// merge the PieChart into the XYChart so that the top-left corner of the PieChart is at
// (leftX, topY) of the XYChart
c.makeChart3().merge(p.makeChart3(), leftX, topY, Chart.TopLeft, 0);