History Dial

History Dial is one of Pablo's display functional units.  It represents its
input parameter as a position of the pointer on a circular dial face.
The dial position is set according to the magnitude of the data value
and the "minimum" and "maximum" data values stored within the functional
unit and configured by the user.

In addition to the displayed value, HistoryDial maintains a "sticky hold"
value.  Colored arrows are used to indicate the latest positions of the dial, 
where the number of such positions maintained is a user-configurable parameter.

Both the minimum and maximum dial values are at 12-o'clock position.


HistoryDial attributes:

	XtNheight	height (in pixels) of the dial
	XtNwidth	width (in pixels) of the dial 
	XtNbackground	the background color of the widget
	XtNhistory	number of "history arrows" to be maintained
	XtNfaceBorderColor   color of the circle which defines the face
	                     of the dial
	XtNneedleColor	color of the interior of the pointer drawn to 
			     represent the dial value
	XtNtickColor	color of the tick lines drawn on the dial
	XtNminValue	minimum data value displayed by the widget
	XtNmaxValue	maximum data value displayed by the widget
