Bug summary

Data 0 cannot be plotted and the corresponding area in the the figure remains transparent, if some data is less than 0.

I tried to trace the problem and find that the reason seems to be a bug in ContourSet._get_lowers_and_uppers.(Line 1065 in contour.py(3.3.4))

    if self.zmin == lowers[0]:
        # Include minimum values in lowest interval
        lowers = lowers.copy()  # so we don't change self._levels
        if self.logscale:
            lowers[0] = 0.99 * self.zmin
        else:
            lowers[0] -= 1

while lowers[0] is the same as self.zmin, it will be reduced a little before using.
However, while lowers[0] is greater than self.zmin ( = -1.7E-13 ), it will not be reduced and remains as it is.
I think the condition should be revised to self.zmin <= lowers[0] from self.zmin == lowers[0].

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

def main():
    z0 = np.array([[0., 0., 2.], [0., 0., 1.], [2., 1., 0.]])
    z1 = np.array([[0., 0., 2.], [0., 0., 1.], [2., 1., -1.7E-13]])

    fig = plt.figure(figsize=(2, 4))

    ax0 = fig.add_subplot(121)
    ax0.contourf(z0, cmap='jet')
    ax0.set_aspect('equal')

    ax1 = fig.add_subplot(122)
    ax1.contourf(z1, cmap='jet')
    ax1.set_aspect('equal')

    plt.show()

if __name__ == "__main__":
    main()

Actual outcome

the left-bottom area in the right figure is not plotted and remains transparent,
just because the data z[2,2] is changed from 0 to -1.7E-13.

Expected outcome

The figure in the right site is expected to be the same as the left one.

Operating system

Windows 10

Matplotlib Version

3.3.4

Matplotlib Backend

module://ipykernel.pylab.backend_inline

Python version

3.8.8

Jupyter version

6.0.1

Other libraries

No response

Installation

conda

Conda channel

No response