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

importnumpyasnpimportmatplotlib.pyplotaspltdefmain():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