Color gradient problem on X-Y chart

I used a top-to-bottom color gradient on an X-Y chart (formatted as a thick
line, with no markers) – lower value line regions were red; middle values
orange and high values, at top of chart, were green. It looked great… until
it was opened on 2 other PCs (same OS & Office version), where the gradient
is somehow corrupted: Instead of the color gradient progressing from the
bottom to top of the chart, it changes too quickly along the line, then
starts to repeat part-way up the line. This pattern continues, resulting in
a mixture of colors at different heights on the chart; so some high values on
the line are green, some red, some orange…

P.S. Dear Microsoft: I’m a power user. I've just switched to 2007 and
HATE having to relearn how to get to all the settings. Also hate dealing with
features that have been removed, yet some of the obvious problems &
limitations of earlier versions have still not been fixed! Enough of the UI
eye candy!! – do you realize how many labor hours you are responsible for
wasting?? OK, rant over!

Paul Prince-Wright