Slightly improved answer:

(If I knew how much I'd turn out to know I should have called myself
earlier!)

The problem arises because Excel wants to recalc the trendline curve using
more-finely spaced points when you tell Excel to draw or print to
higher-resolution device. As a side-effect that obscures the dashed line
effect.

But we can take a copy of the chart image when it's in coarsely-calculated
state (eg: Excel worksheet window at 100%) and paste the image (vector
"Picture" not bitmap) somewhere else, like Word. That prints just fine
(because of coarse Word is not going to recalc the trendline).

Graham

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Graham Wideman
Microsoft Visio MVP
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Book/Tools:
Visio 2003 Developer's Survival Pack
Resources for programmable diagramming at:
http://www.diagramantics.com