How to get matplotlib to place lines accurately?

Question

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?

Answer 1

Consider the following to see what is going on

import matplotlib.pyplot as plt

fig, 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.

Answer 2

The problem is even more notorious when one plots functions which are symmetric with respect to the x axis and slowly approach zero. As for example here:

which is indeed embarrassing if you are telling the reader of a scientific paper that the two curves are symmetric!

I went around this problem by exporting to pdf instead of exporting to png: