Something like this:
if (_dataX[i].Length == _dataY[i].Length)
{
EnumerableDataSource<int> xOneCh = new EnumerableDataSource<int>(_dataX[i]);
xOneCh.SetXMapping(xVal => xVal);
EnumerableDataSource<int> yOneCh = new EnumerableDataSource<int>(_dataY[i]);
yOneCh.SetYMapping(yVal => yVal);
CompositeDataSource ds = new CompositeDataSource(xOneCh, yOneCh);
Action UpdateData = delegate()
{
((PointsGraphBase)plotter.Children.ElementAt(startIndex + i + 1)).DataSource = ds;
};
this.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Normal, UpdateData);
}
Cast it to PointsGraphBase