Use jQuery.isArray() instead of instanceof
jqplot uses instanceof checks to test whether the 'data' parameter is an array. This will not work when the object comes from another DOM (iframe) which is the case when using GWT.
Symptom was reported in the users group here: http://groups.google.com/group/jqplot-users/browse_thread/thread/ca63d796c76b4ab2/f6735acc7df5e478
Suggestion: follow jQuery's coding guidelines for type checks: http://docs.jquery.com/JQuery_Core_Style_Guidelines#Type_Checks