added checks for disposed objects

Change-Id: I6fa21570e361576696f40fb7dbff001b309e8c29
added support for tools that change their list of selectable attributes

Change-Id: Iae860330f3af09a2a6f327a2fc181b7be10450c1
line chart: added support for group-by attributes in attr selector and
renamed xAttributeColumns -> xColumns.
added test cases.

Change-Id: Icb273b7aac082d9a1d6af38b4f7ecc34d309a18d