Thank you very much for the suggestion!
When checking the web.xml syntax for specifying a storage in the CeWolf tutorial (on
http://cewolf.sourceforge.net/), I also stumpled across the session-timeout setting (1 minute), and copied it. That alone helped much, memory consumption built up much more slowly. Seems I went too quickly through the tutorial the first time. However, I still had thousands of ChartImageDefinition objects on the heap, referenced from a large ConcurrentHashMap in the session with String keys representing integer values, like "-1237369985".
However, additionally specifying LongTermSessionStorage in web.xml and a timeout of 10 (seconds) in each cewolf:img tag on my JSP page seems to have eliminated the leak completely. That's so great!