Led

Led is one of Pablo's display functional units.  It represents its
input parameter as a set of levels stacked either horizontally or
vertically.  Individual levels are filled according to the magnitude
of the datum and the "minimum" and "maximum" data values stored within
the functional unit.  The minimum and maximum values may be set by the user
during the configuration process.

In addition to the filled levels, Led maintains a "sticky hold" value.  
A single level is drawn in a specific color to mark the maximum value the
led has obtained during a given period.  This time interval can be set
during the configuration process; the value specified is the number of
previous updates that are considered when determining the sticky value.

Led levels can be filled either with a color or with a pixmap.

Clicking the left mouse button with the cursor inside the led invokes a
callback displaying the current value of the led and also its row and/or
column position (in the case of vector and array layouts).


Functional Unit Inputs:	scalar, vector, array

Led attributes:

	XtNheight	height (in pixels) of the led widget
	XtNwidth	width (in pixels) of the led widget
	XtNstickyHold	number of updates used as history for sticky hold
	XtNlevels	number of stacked levels making up the led
	XtNorientation	direction in which levels are stacked (horizontally
				or vertically)
	XtNshowOutline	draw an outline around the led
	XtNoutlineColor	color of the outline
	XtNnumColors	number of different colors in the color scale used
				to fill levels
	XtNbackground	the background color of the widget
	XtNminValue	minimum data value represented by the widget
	XtNmaxValue	maximum data value represented by the widget
