docx4j can't convert a chart to png, but that doesn't stop you from re-using it in a docx.
To do that, you OLE embed it.
To see what your XML needs to look like, add your chart in Word, save the docx, and look at the results.
This approach makes sense if the docx will be opened by users in Word. If you don't want to do that, but rather want to emit PDF or HTML directly, then you need another approach (since docx4j's PDF/HTML output won't know what to do with it).