Implement picking. This basically involves cloning the code from Gview.cc.

This really needs to be done in an intermediate superclass!
All these changes really in previous revision; just missed typing in comments then.

src/Plot.cc

@@ -136,6 +136,7 @@ void Fl_Plot::draw() {
     }
   }
   glPopMatrix();
+
   /* Draw axes */
 
   draw_axis( x0_, x1_, -.01, -.04, .1, "n", xtitle() );