Working version!

Had to get the canvas object to draw_idle() in order for it
to be refreshed. Now it works great! I might tweak the step size
for the sliders, as 0.1 can end up with weird, not quite complete
plots.

spirographs.glade

@@ -136,6 +136,7 @@
         <child>
           <object class="GtkViewport" id="plotViewport">
             <property name="visible">True</property>
+            <property name="app-paintable">True</property>
             <property name="can-focus">False</property>
             <child>
               <placeholder/>