time series chart in null date

Hi, everyone

Currently using

Crystal Reports 2008 Product: 12.7.0.1983

I make time series chart using a line graph.

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№ X-axis Y-axis

1 2014/1/1 100

2 2014/1/2 20

3 2014/1/3 null

4 2014/1/4 500

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Null becomes 0 when I make an upper list a chart.

I skip a date of null, and may you make a time series chart?

Regrards,

Kano