A "Probe" event is delivered when a mouse pointer moves over a region of
a Graph occupied by a data-element. A ProbeEvent object is sent as an argument
to the ProbeListener's "probeUpdate" method.
ProbeEvent objects will contain all the data associated with the subject
data-element in the form of an instance of a PickInfo class.
"data-elements" are those chart graphics depicting the values found in
the data model. For example the bars in the BarChart are data-elements but
the axis, legend, titles, footnotes, etc are not.
Use the PickInfo property to retrieve the associated data.