If using the community charts (JFreeCharts), here is how you would do this.
You need to add a group to the report. Right click on the root object in the Report Inspector window. Click 'Add Report Group'. Then add a group with the field you're interested in. You don't have to add the header/footer bands.
This also creates the variable for you. It should be named something like 'GROUP_NAME_GROUP_COUNT'
Drag a new chart onto the summary band. (I believe it has to be in a summary band, although I could be wrong). Set the unique identifier (Key Expression) to '$F{CallType}' and the numeric value (Value Expression) to '$V{GROUP_NAME_GROUP_COUNT}')
If this doesn't work then try creating the variable manually. Add a variable, name it 'CallCount'. Set the class to 'java.lang.Integer', calculation to 'Count', and Reset Type to 'Group'. The reset group should match the group you created earlier. The Variable Expression should be the field you're interested in. In this case 'CallType'. Set the numeric value to 'CallType'
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