I would say no, visualization questions should be on topic here. Visualization is a part of scientific computing, after all, and probably the most tangible part at that. Plots and pretty pictures are what you show off when you want to justify why you are spending all this money running giant computers and doing science ;-)
Of course, data visualization questions would be subject to any other rules we come up with. For example, if we (hypothetically) decide questions about software package configuration are off topic, then questions about matplotlib configuration will be off topic. If "what's the best way to X" is decided to be too vague or too subjective, then "what's the best way to visualize X" will be too vague or subjective.
The first section of the FAQfirst section of the FAQ is an ideal place to list this sort of question as being on topic.