Python – plot contours/contours in matplotlib from (x, y, z) datasets

Hi all, I’m

new to programming and I’m trying to do something that might be very obvious, but for me, I can’t figure it out. I have a series of x, y, z data (in my case, corresponding to distance, depth, and pH). I want to use matplotlib to plot contours of z-data (pH) on an xy (distance, depth) grid. Is there any way? Thank you.

The solution will depend on how the data is organized.

Data on a grid of rules

If the x and y data already have a grid defined, they can be easily reshaped to a quadrilateral mesh. For example

#x  y  z
 4  1  3
 6  1  8
 8  1 -9
 4  2 10
 6  2 -1
 8  2 -8
 4  3  8
 6  3 -9
 8  3  0
 4  4 -1
 6  4 -8
 8  4  8 

Can be plotted as > used in contour

import matplotlib.pyplot as plt
import numpy as np
x,y,z = np.loadtxt("data.txt", unpack=True)
plt.contour(x.reshape(4,3), y.reshape(4,3), z.reshape(4,3))

Arbitrary data

(a) If the data is not on a quad grid, data can be inserted on the grid. matplotlib itself provides a way to use matplotlib.mlab.griddata.

import matplotlib.mlab
xi = np.linspace(4, 8, 10)
yi = np.linspace(1, 4, 10)
zi = matplotlib.mlab.griddata(x, y, z, xi, yi, interp='linear')
plt.contour(xi, yi, zi)

(b) Finally, contours can be drawn completely without using a quad grid. This can be done using tricontour

plt.tricontour(x,y,z)

An example comparing the latter two approaches can be found on the matplotlib page