Serdar,
Thanks for your report. The clearChart() function is indeed missing from the calendar code. We'll fix that at least for the next release, along with some other missing methods. Perhaps we will update the latest (upcoming) release as well, if it turns out there is another reason.
I'm not sure there is much need to release resources with the Calendar chart. If you find there are memory leaks, we'd like to know about it, but you might try to reuse existing Calendar chart instances rather than creating new ones when redrawing charts, for example.