Thanks Phil, that will be useful for future reference. I actually did
discover, just now, what my problem was. I had downloaded the data from
another source, and there was a blank cell amongst the data. This caused the
chart to change from an XY to a line chart as there was no "x" for the "y" at
that point. Thought I'd post this as it may be useful to others.

"Phil Preen" wrote:

Is there a max number of data points an XY chart can
use?

I think the maximum is 32000 points per series.