The cell with the formula isn't blank, and "" does not produce a blank.
There is not a formulaic way to simulate a blank, but for many cases, a line
chart or XY chart will treat NA() as a blank. It produces an ugly #N/A error
in the worksheet, but it isn't plotted, and you can pretty up the worksheet
with conditional formatting:

http://contextures.com/xlCondFormat03.html#Errors

- Jon
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Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
http://PeltierTech.com
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"Sandybam" wrote in message
...
I have a chart that references cells with the following formula
=if(a1="","",a1/a2). The formula is used for each month Jan-Dec. I would
like to copy the formula across all months. When I do this my data point
show as zero for months that have not happened yet. What can I do to not
show a line on the chart for a data point with a "" or null value?

Thank you,
Sandy