Adding a line to a PivotChart
 

I'd like to add a line to a column PivotChart representing the upper
quartile of a series. I can calculate the upper quartile in code, but
can't find a way in code to place a line so that it crossed the Y
axis at the upper quartile. I can do this in an Excel chart by adding
and formatting a series, rather than drawing a line; but in a
PivotChart, I'm unable to add a series or a trend line.