Use the drawable-field to display the chart in the report.
Using the drawable-field, then in the chart expression's properties, the property "useDrawable" should be set to true. If the useDrawable property is not present, the default value is true.
The imageurl-field no longer supports rendering the output from a chart expression!
The imageurl-field used to be the prescribed element for chart objects in a JFreeReport. This is no longer the case, in fact the imageurl-field no longer supports displaying the output from a chart expression. You should change any and all reports that attempt to use imageurl-field to use drawable-field.