XL2003: Shape-borders on scatter-series' look wavy

[Holger Gerths]

Dear group,

this question is concerning Excel 2003 on Win XP.
A client sent me a chart-example, where he inserted an ellipse as an
excel-shape on a scatter-series
to highlight a chart-labelling. The ellipse outside the chart is looking
normal, but the one pasted on the data-series
has a wavy border. Using my computer and pasting the same shape into the
chart, the ellipse is looking correctly.
If I give it back to my client, he can see also see the correctly looking
shape in the chart.
My client has Excel 2003 SP2, I have SP3.
Any idea which effect might trigger this?

Thanks in advance, Holger.