cewolf is a tag library for putting charts into JSP pages. It's based on JFreeChart, but you don't need to know that library. You can find a link to an updated version of it (fewer bugs, more features) in my signature.
Jasper is a reporting library; I don't think it works well if all you want is charts in JSP pages.
cewolf is good and is sufficient for basic implementations. For more complex graphs, while you can still use cewolf to draw the charts, it would help if you know the jFreeChart library to constuct the right data model (basically you would have to wrap your data in jfreechart api objects) for cewolf to display the charts.
Last time I used it (some three years back), cewolf stored the generated image in session. Since we made heavy use of charts, the application began running into memory issues. I had to peek inside the code to look up the session attribute name and remove it for every request.