This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I don't see setMinVal and setMaxVal methods in the DatasetProducer class that would accept -and subsequently make use of- those values.
But regardless, cewolf always scales the axes to accommodate the actual values; there is no way to display larger axis value without larger actual data values. I'm not even sure if the JFreeChart library (which is used by cewolf) supports that.
As an aside, you should use the cewolf distribution downloadable from the link in my signature, not the one from SourceForge.