Distinct Count in Stacked Bar Chart

Hi all, 

I have just successfully built a summary report by having distinct count with an artificial batch key (i.e. customer ID + date). For summary report, i could utilize the summarize data section. 

However, in Stacked Bar Chart, in section "Data values on y axis", only numeric type field could be selected. Is there any alternatives to show the distinct count of a batch key in stacked bar? 

Thanks for the help.

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Alex Wong
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