You can register a ProbeListener with a chart so as to be notified
when a the mouse pointer passes over a data-element area. The "Probe" event
will contain all associated data ascribed to the data-element.
"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.