Fix empty plot with drawstyle="steps"

[timhoffm]

Fixes #9597 as suggested there in the comments.

[jklymak]

@timhoffm Can you add a test so this doesn't break in the future? Could probably just check that the call doesn't error...

[jklymak]

@timhoffm Does this make it through to plot, and can a test be added to test_axes.py?

plot(np.array([]), np.array([]), drawstyle="steps")

[timhoffm]

Yes it does. I've intentionally only added unit tests for the pts_to_*step functions. These were the ones failing. We shouldn't add full integration tests for all possible inputs. This really slows the test suite down.

[jklymak]

Ok I’m not sure what the testing philosophy is/should be. To me if it broke once it’ll break again and having a test will stop that. If you can make the test a non-image test I don’t think it takes very long.

[timhoffm]

The things that was broken, were the pts_to_*step functions. And they are tested now, so they can't break again.

Unfortunately, every axes instantiation is quite slow #6664. For a single plot, this doesn't take too long, but it really adds up for several thousand tests.

[jklymak]

I'm not going to hold it up on the testing point.

[jklymak]

Just to carry on this discussion: to me, its the user-facing features that need to keep working. Someone could decide to do away with pts_to_pre etc, and implement steps= in a different way, but the user-experience in plot shouldn't change (or at least we should know it breaks a test). But I'm pretty new here, so please don't take that as our policy!

[anntzer]

I think the test is fine.
Thanks!