How to add a y=x line in a scatterplot with matplotlib

Question

I have an ascii file i with two variables and I want to create a scatterplot and add the line y=x in the scatterplot. The code for creating the scatterplot is below.

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

val1 = np.loadtxt(i, usecols=0)
val2 = np.loadtxt(i, usecols=1)
fig, ax = plt.subplots(1, 1)
fig.set_size_inches(6, 6)
plt.scatter(val1, val2)
plt.savefig(outfile, bbox_inches='tight', format='png', dpi=200)

How to define the plot of line y = x without define the axis scale? Because the scale of the axes is dynamic based on the data. I want to do that for multiple files.

plt.plot(val1, val1) might work

niaei
– niaei

2022-01-12 20:03:56 +00:00
Commented Jan 12, 2022 at 20:03 — niaei
– niaei, Commented Jan 12, 2022 at 20:03
@MSH. No it won't

Mad Physicist
– Mad Physicist

2022-01-12 20:10:00 +00:00
Commented Jan 12, 2022 at 20:10 — Mad Physicist
– Mad Physicist, Commented Jan 12, 2022 at 20:10

Mad Physicist · Accepted Answer · 2022-01-13 06:15:31Z

2

While there are such things as axvline and axhline, and their less popular, but more general, sibling plt.axline. You can specify a pair of points, or a starting point and a slope to draw an infninite line:

plt.axline([0, 0], slope=1)

Another approach is to draw a line in the region of your data:

xmin = val1.min()
xmax = val1.max()
ymin = val2.min()
ymax = val2.max()
xmargin = 0.1 * (xmax - xmin)
ymargin = 0.1 * (ymax - ymax)
margin = 0.5 * (xmargin + ymargin)
cmin = max(ymin, xmin)
cmax = min(xmax, ymax)
plt.plot(*[[cmin - margin, cmax + margin]] * 2)

Both methods are shown below with plt.axis('equal') and the following dataset:

val1, val2 = np.random.uniform([10, 20], size=(100, 2)).T

edited Jan 13, 2022 at 6:15

answered Jan 12, 2022 at 20:09

Mad Physicist

116k29 gold badges202 silver badges292 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

JohanC Over a year ago

What about the (rather new) function ax.axline() which accepts a slope (or two points) as parameter?

Nat Over a year ago

Thank you. However I wanted to plot the line y=x. I tried the code but I didn't;t get the line.

Nat Over a year ago

@JohanC I tried also the ax.axline but the problem is that I do not know the scale of each dataset since I use it for different files

Mad Physicist Over a year ago

@Nat. This plots the line y=x. When you plot anything of the form plt.plot([p1, p2], [p1, p2]), that's clearly along the line y=x

Mad Physicist Over a year ago

@Nat. Are you looking for plt.axis('equal')? I can add that to the answer...

|

Collectives™ on Stack Overflow

How to add a y=x line in a scatterplot with matplotlib

1 Answer 1

10 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

10 Comments

Your Answer

Sign up or log in

Post as a guest

Related