matplotlib 轮廓 plot：得到虚假线

Question

I have the following code to plot the following 2D function:

$$ \frac{x^2 + y^2}{x*y+1} = 4 $$

The code is as follows:

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
xlist = np.linspace(-5.0, 5.0, 100) # Create 1-D arrays for x,y dimensions
ylist = np.linspace(-5.0, 5.0, 100) 
X,Y = np.meshgrid(xlist, ylist) # Create 2-D grid xlist,ylist values
F = (X**2 + Y**2)/(X*Y+1) 
CS = ax.contour(X, Y, F, [4.0],linewidths=0.5, colors='k')

ax.clabel(CS, inline=True, fontsize=10)
ax.grid(b=True, which='major', axis='both') 

I get a spurious lines.

Any help regarding the issue is much appreciated.

Answer 1

I think this is related to the relatively coarse resolution of the image your analyzing.

If I crank the number of points in your calls to linspace , I get a better looking plot:

import matplotlib.pyplot as plt
import numpy as np

fig, axes = plt.subplots(ncols=2, nrows=2, figsize=(8, 8))
for n, ax in zip([50, 100, 500, 1000], axes.flat):
    xlist = np.linspace(-5.0, 5.0, n) # Create 1-D arrays for x,y dimensions
    ylist = np.linspace(-5.0, 5.0, n) 
    X,Y = np.meshgrid(xlist, ylist) # Create 2-D grid xlist,ylist values
    F = (X**2 + Y**2)/(X*Y+1) 
    CS = ax.contour(X, Y, F, [4.0],linewidths=0.5, colors='k')

    ax.clabel(CS, inline=True, fontsize=10)
    ax.grid(b=True, which='major', axis='both') 
    ax.set_title(f"N ={n}")