Piechart

Piechart is one of Pablo's display functional units.  It accepts a vector
of data and represents the individual vector elements as slices of a pie.
The size of a slice is proportional to the size of the data element relative
to the others in the array.  Piechart enforces minimum- and maximum-allowed
magnitudes for incoming data; values outside this range are treated as either
minimal or maximal, accordingly.  The minimum and maximum can be set by the
user as part of the configuration process.

Clicking the left mouse button with the cursor inside the piechart invokes
a callback indicating which slice was chosen, the size of the slice (as a
percentage of the total pie) and also the value of the original scalar datum
represented by that slice.


Functional Unit Inputs:  vector

Piechart attributes:

	XtNheight	height (in pixels) of the piechart widget
	XtNwidth	width (in pixels) of the piechart widget
	XtNbackground	color of the background of the widget drawing area
	XtNgridColor	color of piechart grid lines (not implemented)
	XtNfaceBorderColor	color of the lines used to draw the piechart
				face
	XtNnumColors    default number of colors used by the piechart
	XtNentries	initial number of pie slices
	XtNshowGrid	set to True if a grid is to be drawn; False otherwise
				(not implemented)
	XtNminValue	minimum data value represented by the widget
        XtNmaxValue     maximum data value represented by the widget
