Scatter plot not plotting XY correctly - plots YY rather than XY. Office 2010

Hello,
myself and several colleagues have been having this problem. In Office 2010 (it didn't happen before), when a series becomes too large (I don't know what the threshold is - but only a couple of thousand points), scatter plots break down and no longer plot XvsY, but instead, plot YvsY, despite all the right data being highlighted.
I attach an example, together with a snapshot in case it's not reproduced on other machines. I'm at a total loss to understand this.

Has anyone else had this problem ? does anyone know if MS is aware of it and fixing it ?

Any help or information would be gratefully appreciated.

Rob

Book1.xlsxYvsY.png

Before anyone wastes time.... I think this is my error.
I have a few points with no data, defined as NA(), but there was one lone point (2911 in my example), with no data and no NA(), when you put =NA() in this cell, everything plots ok.
So, apologies if anyone lost time because of this oversight.

Lesson learned: check your data before plotting...
Still, would rather Excel flagged it rather than plotted something like YvsY.