Changing the axis scale in numpy plot

Question

I am drawing a plot based on a numpy array:

A = np.array([[4,5,6],[2,3,6]])

with plt.plot(A) it works fine and draws based on 6 tuples: (0,4), (1,5), (2,6), (0,2) etc.

I want to scale the x-axis though. The units should be divided by 120. So I want to plot:

(0,4), (1/120,5), (2/120, 6), etc.

Is there any easy way to do it, without looping through the array and manually feeding the tuples to the plot?

I have been reading this stackoverflow.com/questions/34080270/… and it seems like I need to create a new matrix with the x axis coordinates — dorien
– dorien, Commented Oct 27, 2016 at 13:20
I ran it and it gave me three plots ((0,4), (1,2)), ((0,5), (1,3)), ((0,6), (0,6)) — kameranis
– kameranis, Commented Oct 27, 2016 at 13:21
Sorry, I should have been more clear. Yes there are three lines drawn in the plot. I want to change the x-axis of the full plot. — dorien
– dorien, Commented Oct 27, 2016 at 13:23

kameranis · Accepted Answer · 2016-10-27 13:23:02Z

1

Specify the x axis

scaling_factor = 120.
x = np.arange(A.shape[0])/scaling_factor
plt.plot(x, A)

answered Oct 27, 2016 at 13:23

kameranis

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7 Comments

Divakar Over a year ago

A.shape[1] probably?

kameranis Over a year ago

Well... I thought so myself, but when I did it, plt complained ValueError: x and y must have same first dimension. So, I'm guessing that something is off. Either the plt.plot(A) or the points the OP gave us.

Divakar Over a year ago

Or with A.shape[1] and then plt.plot(A, x) maybe?

kameranis Over a year ago

nope, that doesn't work either... You would need to transpose A, so scaling_factor = 120. x = np.arange(A.shape[1])/scaling_factor plt.plot(x, A.transpose())

dorien Over a year ago

The original answer with shape[0] seems to work fine for me.

|

Balzola · Accepted Answer · 2016-10-27 13:23:28Z

0

import numpy as np
A = np.array([4,5,6],[2,3,6])

This fails! What is your actual code?

Anyway have you tried providing the x values explicitely to plt.plot():

x = np.arange(3) / 120
plt.plot(x, whatever)

answered Oct 27, 2016 at 13:23

Balzola

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4 Comments

kameranis Over a year ago

There are actually 2 points in each plot, so x = np.arange(2)/120. Also, now, your x is full of zeroes, because integer division. That's why you add . at the end, to make it a float.

dorien Over a year ago

Thanks. A [] was missing. My original code actually reads in the np array from a huge file.

Balzola Over a year ago

@kameranis Python3 is out for 8 years now and does not require the . to prevent integer division.

kameranis Over a year ago

yes, but some people still prefer to write in python2. You could be explicit about it.

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