Rewrite the turtledemo makeGraphFrame method (#104224)

Rewrite the turtledemo makeGraphFrame method (#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.