In the process of penning #150, I noticed that when capturing the test image to an io.Bytes in-memory buffer:

That the format kwarg to savefig isn't used.

Would you be adverse to defaulting to format='png', particularly when writing to a buffer, as (from what I understand) matplotlib will default to the rcParams to select the file-format to save to the buffer for the image, and this could be a source confusion/issues for hashing repeatability for developers when testing.

i.e., what I'm proposing is that a format='png' is injected into the savefig_kwargs, iff not specified by the caller as a kwarg option in the marker decorator.