How to draw a single contour line in Matplotlib

Question

Following code is borrowed from here:

import numpy as np
import matplotlib.cm as cm
import matplotlib.pyplot as plt


delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z)
ax.clabel(CS, inline=True, fontsize=10)
ax.set_title('Simplest default with labels')

The above code produces the contour plot with 7 contour lines. How to draw a single contour line say with the value 0.5?

The output of the above code is:

try [`levels=1`](https://i.stack.imgur.com/WR0RR.png) or [`levels=2`](https://i.stack.imgur.com/8O9TS.png) , in `ax.contour` — Trenton McKinney, Nov 01 '21 at 05:03
or to get rid of contours where Z is less than 0, use `Z[Z<0] = np.nan`. — Trenton McKinney, Nov 01 '21 at 05:09

score 3 · Accepted Answer · answered Nov 01 '21 at 05:08

3

Just add contour value: CS = ax.contour(X, Y, Z, [0.5])

import numpy as np
import matplotlib.cm as cm
import matplotlib.pyplot as plt


delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z, [0.5])
ax.clabel(CS, inline=True, fontsize=10)
ax.set_title('Simplest default with labels')

plt.show()

answered Nov 01 '21 at 05:08

Yaroslav Kornachevskyi

1,160
10
12

So specify `levels`, like the comment says. – Trenton McKinney Nov 01 '21 at 05:10
CS = ax.contour(X, Y, Z, levels = 1 ) ? – Yaroslav Kornachevskyi Nov 01 '21 at 05:13
Yes, `ax.contour(X, Y, Z, levels=[0.5])` (e.g. _`levels`: int or array-like, optional_) – Trenton McKinney Nov 01 '21 at 05:17
I see, it could be added as a positional argument. But specifying levels = 2 just sets quantity of contours. – Yaroslav Kornachevskyi Nov 01 '21 at 05:21
It was just an example, more to confirm that's what to OP was looking for – Trenton McKinney Nov 01 '21 at 05:22
1

@TrentonMcKinney ok, got it – Yaroslav Kornachevskyi Nov 01 '21 at 05:24

How to draw a single contour line in Matplotlib

1 Answers1