Plotting a grid in Python

Question

I'm trying to start 2D contour plot for a flow net and I'm having trouble getting the initial grid to show up properly.

Given the number of columns and the number of rows, how can I write a function that will plot a grid so that all points in the given range appear?

I tried plotting for 4 columns and 3 rows of points by doing this:

r = 3

c = 4

x = [i for i in range(c)]

y = [i for i in range(r)]

plot(x,y,'ro')

grid()

show()

and get this error:

'ValueError: x and y must have same first dimension'

So I tried testing it on a 4x4 grid and got this and I get close to what I want, however it only plots points (0,0), (1,1), (2,2), and (3,3)

However, I also want the points (0,0), (1,0), (2,0), (3,0), (1,0), (1,1)...(3,2), (3,3) to appear, as I will later need to plot vectors from this point indicating the direction of flow for my flow net.

Sorry, I know my terminology isn't that great. Does anyone know how to do this and how to make it work for grids that aren't square?

you don't need to write `x = [i for i in range(c)]`, you can write `x = range(c)` — YXD, Mar 29 '12 at 10:20
Thanks for pointing that out. I did a lot of working beforehand and I think I had c = 4.0 and r = 3.0 and the range() function didn't work for floats. If anyone knows how to solve this problem for floats that would be great too! — bang, Mar 29 '12 at 10:27
Your plot tries to plot a line when X vector are x-axis values and Y vector are y-axis values. Sizes of the two vectors have to match obviously. And that probably is not what is desired output of your code. — Fenikso, Mar 29 '12 at 10:32

score 7 · Answer 1 · edited May 23 '17 at 12:17

You could use itertools.product to generate the desired points.
Use plt.scatter to plot the points
Use plt.quiver to plot the vector field. (Relevant code taken from these SO answers)

import numpy as np
import matplotlib.pyplot as plt
import itertools
r = 3
c = 4
x = np.linspace(0, c, c+1)
y = np.linspace(0, r, r+1)

pts = itertools.product(x, y)
plt.scatter(*zip(*pts), marker='o', s=30, color='red')

X, Y = np.meshgrid(x, y)
deg = np.arctan(Y**3 - 3*Y-X)
QP = plt.quiver(X, Y, np.cos(deg), np.sin(deg))
plt.grid()
plt.show()

enter image description here

score 3 · Accepted Answer · answered Mar 29 '12 at 10:41

3

r = 3
c = 4

x = [i % c for i in range(r*c)]
y = [i / c for i in range(r*c)]

print x
print y

Gives:

[0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]
[0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2]

When used to draw graph as you did it produces desired result.

answered Mar 29 '12 at 10:41

Fenikso

9,251
5
44
72

I believe *Python 3.x* users would use a double slash // in "i / c" to achieve the same result. – John Madden Feb 02 '21 at 18:30

score 2 · Answer 3 · answered Mar 29 '12 at 10:32

The first two arguments specify your x and y components. So the number of points must match. I think what you want is something like:

from itertools import product
import matplotlib.pyplot as plt

points = np.array(list(product(range(3),range(4))))

plt.plot(points[:,0],points[:,1],'ro')
plt.show()

Plotting a grid in Python

3 Answers3