By default, matplotlib plot can place lines very inaccurately.

For example, see the placement of the left endpoint in the attached plot. There's at least a whole pixel of air that shouldn't be there. In fact I think the line center is 2 pixels off.

How to get matplotlib to draw accurately? I don't mind if there is some performance hit.

Inaccurately rendered line in matplotlib plot:

Inaccurately rendered line in matplotlib plot - detail magnified:

This was made with the default installations in Ubuntu 16.04 (Python 3), Jupyter notebook (similar result from command line).

Mathematica, for comparison, does subpixel-perfect rendering directly and by default: Why can't we?

Consider the following to see what is going on

import matplotlib.pyplot as pltfig, ax = plt.subplots()
ax.plot([1, 2, 3], [4, 5, 4], clip_on=False, lw=5, alpha=.5)
ax.set_xlim([1, 3])fig.savefig('so.png', dpi=400)

You can also disable pixel snapping by passing snap=False to plot, however once you get down to placing ~ single pixel wide line, you are going to have issues because the underlying rasterization is too coarse.