Please indicate the issue being fixed, and include code snippet that you are testing with.

The issue being addressed is handling automin and automax when yscaled len = 1, in the def _proportional_y function in test/colorbar.py.

 if self._extend_lower() or self._extend_upper():
            automin = yscaled[0]
            automax = yscaled[0]
            if len(yscaled) > 1:
                automin = yscaled[1] - yscaled[0]
                automax = yscaled[-1] - yscaled[-2]

The (failing) test generating the warning is:

def test_contour_uniformfield_colorbar():
    # Smoke test for issue
    fig, ax = plt.subplots()
    with pytest.warns(Warning) as record:
        cs = ax.contour([[1, 1], [1, 1]])
    assert len(record) == 1
    fig.colorbar(cs, ax=ax)

Looking at the stack trace, the warning seems to come from self.ax.set_ylim(lower, upper) where lower == upper. Looking for other instances of this particular warning (such as here), it appears to come from setting the min and max y values to be the same. This makes me question what the expected behavior from the test should be.

The original code for reproduction of this issue is

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
cs = ax.contour([[1, 1], [1, 1]])
fig.colorbar(cs, ax=ax)
plt.show()

However, this seems to yield the following TypeError

TypeError: Input z must be 2D, not 1D

The same TypeError also occurs if we change

cs = ax.contour([[1, 1], [1, 1]])

to

cs = ax.contour([[1, 2], [3, 4]])

This TypeError is resulted from lib/matplotlib/contour.py, line 1526, in _initialize_x_y, which makes me think that rather than a colorbar problem, the root of this issue lies in how contour works. What is the expected behavior of the original code for reproduction? If we aren't supposed to call Fontour on input like above in the first place, how is Colorbar supposed to behave and why does the the API warning even matter?

I was able to suppress the failed test case with the following change to @quinnah's test case.

def test_contour_uniformfield_colorbar():
    # Smoke test for issue
    fig, ax = plt.subplots()
    with pytest.warns(Warning) as record:
        cs = ax.contour([[2, 2], [2, 2]])
    assert len(record) == 1
    try:
        fig.colorbar(cs, ax=ax)
    except:
        pass

Hi @quinnah - is this ready for review? Do you need help with moving this forward? Cheers!