fix error on changing dataset after first init

Hey,

had one error when changing dataset after first init:


    Error: [$rootScope:infdig] 10 $digest() iterations reached. Aborting!
    Watchers fired in the last 5 iterations: [["dataset; newVal: [{\"color\":\"#edab98\",\"label\":\"Prospects 10     
    %\",\"data\":[[\"2013-06-30T22:00:00.000Z\",0],[\"2013-07-31T22:00:00.000Z\",0],[\"2013-08-
    31T22:00:00.000Z\",0],[\"2013-09-30T22:00:00.000Z\",0],[\"2013-10-31T23:00:00.000Z\",0],[\"2013-11-
    30T23:00:00.000Z\",0],[\"2013-12-31T23:00:00.000Z\",0],[\"2014-01-31T23:00:00.000Z\",0],[\"2014-02- [...]


This PR fixes this :-)