As a user new to prawn, I faithfully started to play with prawn by copying the code in the examples in the manual into my own little test app, to see for myself the effects of the code. One of the first Basic Concepts, origin.rb, calls stroke_axis which fails mightily (and mysteriously, since the error message doesn't give a clear indication of what happened), since stroke_axis is defined in example_helper.rb, which I hadn't required in my test app.
Add note on stroke_axis to manual introduction.
Thanks for the comment. I've added a note to the manual's introduction.