I havent tried charts with princexml, but flying-saucer has capability to do custom rendering. So if you want, you can use a service to get image of your chart and then render that in the pdf as image.
You can try using FlyingSaucer (https://code.google.com/p/flying-saucer/) It takes in HTML and gives PDF. It also supports print related CSS3 properties and its open source. May be it cannot compete with Prince, but it is a good open source alternative