Axolot Data

I'm trying to set the marker style of a series like so:

vSeries := Series[0];
with vSeries do begin
HasDefDataPoint := TRUE;
DefDataPoint.MarkerFormat.Automatic := FALSE;
DefDataPoint.MarkerFormat.Size := 20;
DefDataPoint.AreaFormat.ForegroundColor := xcYellow;
end;

If I do this, save the workbook, then open it, Excel throws an error: "File error: data may have been lost.". The workbook then opens, but the series referenced above has a marker size of 2pts, and black fore/background.

What is the proper way of setting the marker size?

thanks,

Bob M.