If you've fixed the number of digits using only number formats, the values remain
numeric, and they should chart just fine. This means selecting the range, going to
the Formatting menu and selecting Cells, and on the Number tab, selecting Number in
the category list, and setting a number of digits.

I'm not familiar with a "fixed function". If it's someone's user defined function
that converts the number to text and trims it to so many characters, that someone is
playing a cruel trick.

- Jon
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Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
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