Some widget sets have a widget particularly for this purpose -- a WorkSpace
or DrawingArea which doesn't display anything but lets your Xt application
know when it has been re-exposed, resized, and when it has received user key
and mouse input.

The best thing to do for other widget sets -- including the Athena set 
-- is to create or obtain such a widget; this is preferable to drawing into a 
core widget and grabbing events with XtAddEventHandler(), which loses a number 
of benefits of Xt and encapsulation of the functionality .  

The publicly-available programs xball and xpic include other versions.  The
Display widget in the XG library (libXG-2.0.tar.Z on ftp.x.org) provides a
generic way of drawing graphics in a widget.

The Athena Widget manual (mit/doc/Xaw/Template in the R5 distribution,
xc/doc/specs/Xaw/Template in the R6 distribution) includes a tutorial and
source code to a simple widget which is suitable for use.

The Free Widget Foundation set contains a Canvas widget.

An Xt Canvas widget by Jean-Michel Leon (leon@sophia.inria.fr) is intended to
provide graphical display and direct manipulation services for Motif and Xaw
clients. Available from ftp://avahi.inria.fr/pub/widgets/,
ftp.x.org:/contrib/widgets/motif/canvas-widget-1.7.tar.gz.

The Knvas widget is intended to supply graphical display and direct
manipulation services for Xaw or Xm applications. Source is on
ftp://avahi.inria.fr/pub/widgets/ and
ftp.x.org:/contrib/widgets/motif/canvas-widget-1.7.tar.gz.  Info:
http://zenon.inria.fr:8003/~leon/widgets/canvas.html .