Add Series to a chart failure and Disappearing Series
 

I have an Excel 2000 macro that puts several series of XY data on a
scatter chart, and it then computes XY data too add a final series to the
chart. I originally tried to use the Add method of the SeriesCollection
object to add the final series. Since the number of points in the final
series is computed and is not known prior to run time, I counted the rows and
used:
Set rng = Sheets("New Data"). _
Range ("A1").Resize(norws,2)
rng.Name = "NewSer"
to name the range.
I then used the named range in the Source parameter of the Add method.
Instead of using the X data from the named range, the point numbers were
used. The Y values were correct. For example, if there were 8 points in
the new series with XValues ranging from 2.1 to 67.5, the XValues used by
Excel were the integers 1 to 8. I am guessing that Excel decided the new
series was a Line format instead of XY format. When I repeated code that
the chart was XYScatterLine, I got the Error Message "Parameter Not Found"
for the named range, "NewSer".
I decided to try the NewSeries method and assign the Values and XValues
as named ranges. This method works fine except that the last series of the
original data occasionally disappears when the new (final) series is added.
If I step through the code, the code always works as intended.
Can anyone explain this behavior for both the Add method and the New
Series method and suggest solutions?