Jon Peltier has instructions for this in his Excel Charting FAQ article:
http://pubs.logicalexpressions.com/Pub0009/LPMArticle.asp?ID=209#jon024
> How do I choose every ninth row to be my Y data on a
> chart? I have 15,000 rows so picking one individually
> will be time consuming.

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Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html