This adds a new histogram tab to the graduated renderer, which
shows an interactive histogram of the values from the assigned
field or expression. Class breaks can be moved or added using
the histogram widget.
A base class, QgsHistogramWidget, has been created to display
histograms for a field or expression. In future this could be
used to show a histogram within a "selection statistics" panel.
Sponsored by ADUGA (http://www.aduga.org)