Charts created with BIRT are saved as images in the format specified in the very first step of the chart wizard, named "Select Chart Type". The default resolution of the produced images is usually quite low and the chart quality might be unsatisfying for example in a PDF document.
I consulted the www.birt-exchange.com web site for help on this issue and I extrapolated the following instructions, which seem to work.
For the general report document:
- Go to the "Properties" panel below
- Select the "Advanced" tab
- Assign the following value
reportContext.getAppContext().put("CHART_RESOLUTION", "300" );
to the property "Before factory".
See the picture below, where I used a 300 dpi resolution.