Using matplotlib backend: MacOSX
これエラーメッセージじゃないです。
A lot of documentation on the website and in the mailing lists refers to the “backend” and many new users are confused by this term. matplotlib targets many different use cases and output formats. Some people use matplotlib interactively from the python shell and have plotting windows pop up when they type commands. Some people embed matplotlib into graphical user interfaces like wxpython or pygtk to build rich applications. Others use matplotlib in batch scripts to generate postscript images from some numerical simulations, and still others in web application servers to dynamically serve up graphs.
matplotlibがいろんなアプリケーションやPythonシェル等で使われるので明示的にbackendを何に設定しているか教えてくれているだけです。
matplotlib - Usage What is a backend?