Rediscovering Schelter's direction field script

Hi,
after talking to James Amundson in Metz, I came to the conclusion that
I'd have to write my own direction-field-plot function.  As I was
working on it, I accidentally ran into a beautiful tcl script (plotdf)
which comes included with Openmath's omplotdata

If you haven't seen it, put the following somewhere where Maxima can
find it (i.e. ~/.maxima/plotdf.lisp)

(defun $plotdf()
(show-open-plot (with-output-to-string (st ) (format st "~%{plotdf}~%} ~%"))))

then do a load("plotdf") and lunch it with plotdf()
You can click on a point to trace an integral curve and change the
direction field, and the options, in the config menu.

I'm now improving $plotdf() to accept different equations and options,
but if any experienced Maxima-Lisp programmer wants to jump in, that
would save me a lot of time :-)

It would be nice that some function was added to src/plot.lisp to let
users access that functionality in an easy way.

Cheers,
Jaime