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Changing Type Property of Data
Series 
The series value attributes Series1Value and Series2Valueshould have the type property double. You have to change them from their current properties to the type property double.
The data series should have the data type double. You have to change the current data type string to the data type double.
Change the data type of the first series as follows:
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1. In the View Designer window click Series1Value.
2. In the Properties window change string to double.
Change the data type of the second series Series2Value as with the first series.
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The data series now have the correct type property. The Properties window should appear as below:

