TODO

* An interactive plot viewer.

* Add a mechanism to the .plot method that facilitates the creation of the
  legend:

    w.plot (x1,y1;color=3,label="First curve");
    w.plot (x2,y2;color=4,sym="triangle",label="Triangles");
    w.legend(0.2, 0.8; world00);

* A .shade_between method that shades between two curves:

   private define shade_between_curves (w, x, ylo, yhi)
   {
      variable x = [x, reverse(x)];
      variable y = [ylo, reverse(yhi)];
      w.shade_region (x, y; __qualifiers);
   }

* Various time/date related tic labels.

* Additional flexibility for tic labels-- perhaps via a callback
  function.  Perhaps time/date labels could be implemented via such
  callbacks.

* allow to set only some world coordinate limits,
  and have the others be determined automatically,
  in analogy to, e.g.,  isis> xrange(xmin, );

* connection between world coordinates and size of plot box
  to have several plots with different ranges, but same scales

* duplicate method for XFig_Object's
-> allow xfig_multiplot to operate on copies of plots


* define an empty picture object that could be returned
  by xfig_new_text when called with NULL or ""