Things to fix:

xfig_plot_points plots "points" whose size is specified by a previous call
to xfig_plot_set_point_size.  The points are n-sides polygons that are
filled using the current plot line color (xfig_plot_set_line_color).

How can these objects be obtained for use in a legend?
Ideas:

   1.  Have xfig_plot_xxx cache the object being plotted into some
       sort of array that can be accessed by a subsequent legend
       command.

   2.  In a call to plot_lines or plot_points, allow the user to specify the
       name of the "curve".  Then plot_legend could be made transparant to 
       the user by having it look at the named curves.
