How to plot files with numpy?

Question

I have a .dat file that contains two columns of numbers so it looks something like this:

111    112
110.9  109
103    103

and so on.

I want to plot the two columns against one another. I have never dealt with a .dat file before so I am not sure where to start.

So far I figured out that numpy has something I can use to call.

data = numpy.loadtxt('data.DAT')

but I'm not sure where to go from here. Any ideas?

so then you have a 2d array of points ... this has nothing to do with *.dat file could be anything *.txt would work exactly the same... your real question is "How Do I plot a numpy array?" — Joran Beasley
– Joran Beasley, Commented Sep 7, 2012 at 4:35
It's easy in gnuplot ;^). plot 'yourfile.dat' u 1:2 (but of course, that doesn't address the actual question ...) — mgilson
– mgilson, Commented Sep 7, 2012 at 4:35
You can use Scavis that interfaces NumPy (or JNumeric in Java) as explained in the Scavis manual — user3247168
– user3247168, Commented Jan 29, 2014 at 3:36
Possible duplicate of Matplotlib basic plotting from text file — Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com, Commented Aug 29, 2016 at 18:55

bmu · Accepted Answer · 2012-09-07 17:13:26Z

15

Numpy doesn't support plotting by itself. You usually would use matplotlib for plotting numpy arrays.

If you just want to "look into the file", I think the easiest way would be to use plotfile.

import matplotlib.pyplot as plt 

plt.plotfile('data.dat', delimiter=' ', cols=(0, 1), 
             names=('col1', 'col2'), marker='o')
plt.show()

You can use this function almost like gnuplot from within ipython:

$ ipython --pylab
...
...
In [1]: plt.plotfile('data.dat', delimiter=' ', cols=(0, 1), 
...                  names=('col1', 'col2'), marker='o')

or put it in a shell script and pass the arguments to it to use it directly from your shell

plotfile_example

edited Sep 7, 2012 at 17:13

answered Sep 7, 2012 at 16:51

bmu

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1 Comment

Dax Feliz Over a year ago

Thanks a lot. This definitely does the job!

Joran Beasley · Accepted Answer · 2012-09-08 17:55:54Z

4

import numpy as np
import matplotlib.pyplot as plot
#data = np.loadtxt("plot_me.dat")
#x,y=np.loadtxt("plot_me.dat",unpack=True) #thanks warren!
#x,y =  zip(*data)
#plot.plot(x, y, linewidth=2.0)
plot.plot(*np.loadtxt("plot_me.dat",unpack=True), linewidth=2.0)
plot.show()

[Edit]Thanks for the tip i think its as compact as possible now :P

plot 1

If you want it to be log10 just call log10 on the nparray)

plot.plot(*np.log10(np.loadtxt("plot_me.dat",unpack=True)), linewidth=2.0)

log10

edited Sep 8, 2012 at 17:55

answered Sep 7, 2012 at 4:40

Joran Beasley

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3 Comments

Warren Weckesser Over a year ago

You can make the code even more concise by using the unpack keyword in loadtxt: x, y = np.loadtxt('plot_me.dat', unpack=True)

Dax Feliz Over a year ago

Thanks a lot! Do you know how I could take the log_10 of these columns?

Joran Beasley Over a year ago

I think just plot.plot(*np.log10(np.loadtxt("plot_me.dat",unpack=True)), linewidth=2.0)

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