Either have the cell return the NA() error as in:
=IF(A2=1,A2*2,NA())
if using formulas, or if the cells truly are empty

Under Tools-Options - Charts, change the way empty cells are handled.

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Best Regards,

Luke M
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"Tom" wrote:

On a line graph not all my values are numeric. Excel is plotting these
letters at 'zero'. What I prefer is for excel to put in gaps instead. How can
I achieve this?