I have just installed 4.2 (build #5891 [13-Feb-2013 22:17] ) and had this problem with the burndown chart.
I read a post in http://youtrack.jetbrains.com/issue/JT-18150
had this fix on build 6022 but the release notes on 4.2 says this has already been fixed which confused me.
I am using the standalone version with 100 user license hosted on our server.
I appreciate any help you can extend.