There are hundreds of plotting libraries in Python, they offer amazing amount of functionality but at the same time this entire area in Python is a huge mess. I would not venture into choosing a handful of them and trying to make them integrate into Slicer’s GUI. There can be some implementation difficulties, too, as these packages often include interactive GUI components that may conflict with Slicer’s GUI components. For example, matplotlib’s Qt GUI is probably incompatible with Slicer’s Qt version (it should still be possible to use matplotlib’s "agg’ backend and display the plot as a rendered image).
For now, I would keep relying on Slicer’s very sophisticated 2D and 3D visualization capabilities and somewhat less sophisticated interactive 2D plotting (which is very fast and directly can visualize plots from Slicer’s VTK-based data storage objects). We could improve plotting API in Slicer to make it a bit more user friendly (slicer.util.plot() is an attempt towards that - see the example above).
If somebody wanted to experiment with third-party plotting then that would be great. It would be interesting to learn about the experiences. If results are very promising then maybe some helper/adapter functions could be added to Slicer that would help using some plotting packages.