Why Do Matplotlib Contour Labels Make Contours Disappear?

The sample data is generated as follows, import matplotlib as mpl print(mpl.__version__) # 3.3.3 import matplotlib.pyplot as plt import numpy as np def f(x, y=0): return np.pi

Solution 1:

It is indeed a problem of the log axes, especially around the asymptote zero. However, why not defining the log axes before plotting, so matplotlib can take this into consideration when plotting?

import matplotlib as mpl
print(mpl.__version__) # 3.3.3
import matplotlib.pyplot as plt
import numpy as np

def f(x, y=0):
    return np.piecewise(x, [x < 1, np.logical_and(1 <= x, x < 10), x >= 10], [lambda x: 0, lambda x: (x - 1) / 9 * 1000, lambda x: 1000])

x = np.logspace(-5, 5, 100)
y = np.logspace(-5, 5, 100)
X, Y = np.meshgrid(x, y)
Z = f(X, Y)

fig, ax = plt.subplots(figsize=(5, 3), dpi=120)
ax.set_xscale('log')
ax.set_yscale('log')
cr = ax.contour(X, Y, Z, levels=3, colors='black')
ax.clabel(cr, inline=True, fontsize=8, fmt='%d')

plt.show()

Sample output: