This is because you are using a notebook and the inline backend is expanding the figure to fit around the line with the padding you found added on. See https://matplotlib.org/devdocs/users/explain/figure/figure_intro.html#notebooks-and-ides. I guess dpi_scale_trans is working relative to the expanded figure and transFigure is working relative to the original.

If instead you save to a file with

plt.savefig('test.png')

you get

Note that, unlike your "expected outcome" above, here the line goes right to the corners.

@rcomer, thanks for your answer!

I tested a lot and found that matplotlib inline uses savefig with an option bbox_inches='tight'. I tested this:

fig = plt.figure(facecolor='#ccc')
ax = fig.gca()
ax.set_axis_off()
line_1, = plt.plot([0, 1], [0, 1], transform=fig.transFigure, clip_on=False, lw=2, c='blue')
ofs = 0
# uncomment line below for workaround:
# ofs = plt.rcParams['savefig.pad_inches']
fig_w, fig_h = fig.get_size_inches()
line_2, = plt.plot([0+ofs,fig_w+ofs], [0+ofs,fig_h+ofs], transform=fig.dpi_scale_trans, clip_on=False, lw=2, c='black')
#plt.show()
plt.savefig('test.png', bbox_inches='tight')

And test.png now is:

So now I suggest this is definitely is a Matplotlib bug.

Please see the pad_inches parameter of savefig. https://matplotlib.org/stable/api/_as_gen/matplotlib.figure.Figure.savefig.html#matplotlib.figure.Figure.savefig

@jklymak, sorry, I don't know how this should resolve this problem. Please explain this in more detail if you could.

I already use these pads in my code as a workaround. But I wonder, why fig.transFigure don't require to shift plotted line.

Because bboxinches=tight changes the size of the figure and adds a pad around the visible artists. The line in figure space then gets stretched.

The problem is that you are asking for two mutually exclusive things: the plot commands ask for the line to start right in the corner of the figure whereas saving with bbox_inches='tight' and the default pad asks for a 0.1 inch gap between the line and the edge of the figure.

@rcomer, I'm just wondering now, why this default pad make line_1 with fig.transFigure transform to be shifted from the left bottom corner, but not line_2 with ig.dpi_scale_trans transform. If you could explain this in more detail, that would be great. I couldn't find anything about it in the documentation, so this seems like an unexpected Matplotlib behavior.

@jklymak, Okay, but why is only one of the lines displaced relative to the lower left corner? I couldn't find anything about this in the Matplotlib documentation.

@sindzicat if you are really keen, the code that makes the adjustment for bbox_inches is here. There is some explicit handling for transFigure, but not for dpi_scale_trans. I do not claim to understand everything this function does.

@rcomer, many thanks! I tried to find this myself, but without success.

I found a way to make code work:

ax.plot([0, 6.4], [0, 4.8], transform=(fig.dpi_scale_trans+mpl.transforms.ScaledTranslation(0,0,fig.transFigure)), clip_on=False, c='blue')

I've added ScaledTranslation to fig.dpi_scale_trans. I'm not sure why this works, though. I feel like there are a few things missing from the Matplotlib documentation.

In my opinion, if you want to mess with transfigure and trans_fig_dpi you should avoid using bbox_inches="tight" which changes the size of the figure on you.

I should add that it's iPython that makes the inline backend use bbox_inches='tight' by default.