Created attachment 928406 [details]
screen shots showing the issue

Description of problem:
When trying to generate the graph for the metric for the time period when there is no collected data, Globally uncaught exception will be shown in the JBoss UI and the following error will be shown in the Message Center:

****
fileName: http://bkramerlt.usersys.redhat.com:7080/coregui/org.rhq.coregui.CoreGUI/C5C5B3F91D68ACB5BF0139DFBBE3299F.cache.html
 lineNumber: 94495: lowestValue is null
   at Unknown.$getJsonMetrics(Unknown Source)
   at Unknown.getJsonMetrics(Unknown Source)
   at Unknown.$drawJsniChart_0(Unknown Source)
   at Unknown.run_26(Unknown Source)
   at Unknown.fire_1(Unknown Source)
   at Unknown.anonymous(Unknown Source)
   at Unknown.apply(Unknown Source)
   at Unknown.entry0(Unknown Source)
   at Unknown.anonymous(Unknown Source)
   at Unknown.anonymous(Unknown Source)
****

Version-Release number of selected component (if applicable):
JBoss ON 3.2.1 and 3.2.2


How reproducible:
Always

Steps to Reproduce:
1. Navigate to Metrics in Monitoring tab;
2. Open the graph (make sure that this is newly added resource so only one-two days of metric is collected for this resource) and press 6m to view 6 months graph


Actual results:
Globally Uncaught Exception will be shown in JBoss ON UI and exception:

*****
fileName: http://bkramerlt.usersys.redhat.com:7080/coregui/org.rhq.coregui.CoreGUI/C5C5B3F91D68ACB5BF0139DFBBE3299F.cache.html
 lineNumber: 94495: lowestValue is null
   at Unknown.$getJsonMetrics(Unknown Source)
   at Unknown.getJsonMetrics(Unknown Source)
   at Unknown.$drawJsniChart_0(Unknown Source)
   at Unknown.run_26(Unknown Source)
   at Unknown.fire_1(Unknown Source)
   at Unknown.anonymous(Unknown Source)
   at Unknown.apply(Unknown Source)
   at Unknown.entry0(Unknown Source)
   at Unknown.anonymous(Unknown Source)
   at Unknown.anonymous(Unknown Source)
*****
will be logged in Message Center.

No exception will be logged in the server.log file.

Expected results:
Empty graph is shown and no exception is thrown.

Additional info:
Run the same test in JBoss ON 3.2.0 but no exception was thrown.