I have written a beginner's tutorial on the wx module.
It starts with a simple Erlang module that runs in the shell. I walk through creating the window, adding wxwidgets, creating monitors to listen to user activity, and building a loop function that connects the window to the Erlang code.
There is a commented wx_object behaviour.
There are examples of using device contexts to draw graphics.
Click here: http://wxerlang.dougedmunds.com to go to the site.