Rewrite the turtledemo makeGraphFrame method (GH-104224)
Replace `self._canvas` and `self.scanvas`, both bound to `canvas`,
with `self.canvas, which is accessed in other methods.
Replace `_s_` with `screen` and `_s_._canvas` with `canvas`.
Add a comment explaining the unorthodox use of
function turtle.Screen and singleton class turtle._Screen.
(cherry picked from commit 96f95df48e)
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
10ee19b737