Bug report

Bug summary

If I attempt to make a contour plot with data that is entirely below the lowest level, and the data has a field of zeros around it, I get an unexpected contour between the zeros and anything positive.

Code for reproduction

importnumpyasnpimportmatplotlib.pyplotasplty=np.linspace(-10,10,20)x_2d,y_2d=np.meshgrid(y,y)data=np.cos(x_2d*np.pi/10.)+np.cos(y_2d*np.pi/10.)np.place(data,data<0.,0.)levels= [3,6]colors= ['b','r']plt.contour(data,levels=levels,colors=colors)plt.show()

Actual outcome

Expected outcome

The plot should be blank, which is what happens if the call tonp.place is skipped.

Matplotlib version

• Operating system: RHEL7.9
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): 3.4.1
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.8.8
• Jupyter version (if applicable): N/A
• Other libraries: numpy 1.20.2