Here is a patch that allows gnuplot to generate tk canvas widget commands.
It does not yet support replot, but I'll probably get to it (it's not hard,
gnuplot has a really nice output interface!).

To use it, build a gnuplot after patching the gnuplot-3.5 sources with...
# patch -p0 < gnuplot-tkcanvas.diff

Then, from within gnuplot...

gnuplot> set term tkcanvas
gnuplot> set output "plot.file"

From wish, 

% source plot.file
% canvas .c
% pack .c
% gnuplot .c

That's it.  The code generated by gnuplot makes a tcl proc called gnuplot
that takes the name of a canvas as it's argument.  When the proc is called,
it clears the canvas, finds the size of the canvas and draws the plot in it
scaled to fit.

Let me know if you fine this useful (I use it with octave, BTW).

D. Jeff Dionne
Jeff@EE.RyeHam.EE.Ryerson.Ca