You can used code similar to below, based on the QCSPCChart for WPF ParetoDiagram example program, (file Windows.xaml.cs) to install a mouse up event handler, and then to query the ParetoChart tooltip object for the data values.
public Window1()
{
InitializeComponent();
pc = new ParetoPlot(spcChart1);
spcChart1.MouseUp +=new MouseButtonEventHandler(spcChart1_MouseUp);
spcChart1.PreferredSize = new Size(600, 400);
}
private void spcChart1_MouseUp(Object sender, MouseButtonEventArgs e)
{
NearestPointData npd = spcChart1.Datatooltip.GetNearestPoint();
int barindex = npd.NearestPointIndex;
double barYvalue = npd.NearestPoint.Y;
double barXvalue = npd.NearestPoint.X;
string[] labelstrings = spcChart1.XAxisLab.GetAxisLabelsStrings();
string labelstring = labelstrings[barindex];
}