Constructs a new event. The source is either the dataset or the
org.jfree.chart.plot.Plot class. The dataset can be
null (in this case the source will be the
org.jfree.chart.plot.Plot class).
Parameters: source - the source of the event. Parameters: dataset - the dataset that generated the event (nullpermitted).
Returns the dataset that generated the event. Note that the dataset
may be null since adding a null dataset to a
plot will generated a change event.
The dataset (possibly null).