Python: x-y-plot with matplotlib


Question

I want to plot some data. The first column contains the x-data. But matplotlib doesn't plot this. Where is my mistake?

import numpy as np
from numpy import cos
from scipy import *
from pylab import plot, show, ylim, yticks
from matplotlib import *
from pprint import pprint

n1 = 1.0
n2 = 1.5

#alpha, beta, intensity
data = [
    [10,    22,     4.3],
    [20,    42,     4.2],
    [30,    62,     3.6],
    [40,    83,     1.3],
    [45,    102,    2.8],
    [50,    123,    3.0],
    [60,    143,    3.2],
    [70,    163,    3.8],
    ]

for i in range(len(data)):
    rhotang1 = (n1 * cos(data[i][0]) - n2 * cos(data[i][1]))
    rhotang2 = (n1 * cos(data[i][0]) + n2 * cos(data[i][1]))
    rhotang = rhotang1 / rhotang2
    data[i].append(rhotang) #append 4th value

pprint(data)
x = data[:][0]
y1 = data[:][2]
y3 = data[:][3]
plot(x, y1, x, y3)
show()

EDIT: http://paste.pocoo.org/show/205534/ But it doesn't work.

1
3
7/26/2016 4:48:17 AM

Accepted Answer

x = data[:][0]
y1 = data[:][2]
y3 = data[:][3]

These lines don't do what you think.

First they take a slice of the array which is the whole array (that is, just a copy), then they pull out the 0th, 2nd or 3rd ROW from that array, not column.

You could try

x = [row[0] for row in x]

etc.

2
4/23/2010 2:56:46 PM

You can do this by converting data to a numpy array:

data = np.array(data) # insert this new line after your appends

pprint(data)
x = data[:,0]    # use the multidimensional slicing notation
y1 = data[:,2]
y3 = data[:,3]
plot(x, y1, x, y3)

A few additional points:

You can do the calculation in a more clear and vectorized way using numpy, like this

data = np.array(data)
rhotang1 = n1*cos(data[:,0]) - n2*cos(data[:,1])
rhotang2 = n1*cos(data[:,0]) + n2*cos(data[:,1])
y3 = rhotang1 / rhotang2

As you wrote it, your calculation may not give what you want since cos etc take radians as their inputs and your numbers look like degrees.


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