After some test and investigations I found a solution:
BIRT offers also charts. That allows to create the charts directly in BIRT. These charts do fit better to the report.
But as long as charts are used both in Knime and the report it is double work.
You have already found one work-around of using the charting directly in BIRT. Alternatively, you can use KNIME charts exported into BIRT as SVG. There are step-by-step instructions on how to make this work in the description of the Export Image As option in the Image to Report node description, for example here: https://kni.me/n/jbSKGWgB90YaR_66