Plotting is a killer feature of Sage, IMO. It's a wrapper over matplotlib, and eliminates reams of boilerplate. In my experience, it's not just easier to use, but also easier to navigate the Sage docs than figure out the right way to do something in matplotlib.
I'm a weensy bit biased; I probably took part in conversations around the plotting interface as it was taking shape, and got familiar with that before learning matplotlib in earnest.
> It's a wrapper over matplotlib, and eliminates reams of boilerplate.
What happened is that in 2006 I hired Alex Clemesha to implement a "Mathematica-like API on top of matplotlib for Sage", and he spent a year doing so. Then Robert Bradshaw (lead dev of Cython) and Carl Witty wrote some really sophisticated just in time compiler code to make it more efficient to evaluate symbolic expression at many floating point values, which helps in plotting (especially for 3d plots). Over many years after that other people filled in little missing pieces of functionality and exposed more of Matplotlib's functionality, and also just added a bunch of new functionality (e.g., for drawing gridlines, 3d implicit plots, etc.) that wasn't available in the Python ecosystem.
I'm a weensy bit biased; I probably took part in conversations around the plotting interface as it was taking shape, and got familiar with that before learning matplotlib in earnest.