By default, matplotlib shows a figure in a window with a rudimentary user interface. This interface allows you to save the figure to a file. Although it is a reasonable approach for prototyping, it is not convenient in several common usage cases. For instance, you might want to generate a dozen pictures to be included on an automatically generated report. You might want to generate one picture per input file as a batch processor. matplotlib allows you to directly save the figure to a picture file with great flexibility.

To get started, we are going to see how to output a figure to a PNG file. A PNG file is ideal for a bitmap output, and it is also the de-facto standard for bitmap pictures. It's a well-supported standard; it relies on a lossless compression algorithm (thus avoiding unsightly compression artifacts), and handles transparency.