plot data from CSV file with matplotlib

Question

I have a CSV file at e:\dir1\datafile.csv. It contains three columns and 10 heading and trailing lines need to be skipped. I would like to plot it with numpy.loadtxt(), for which I haven't found any rigorous documentation.

Here is what I started to write from the several tries I found on the web.

import matplotlib as mpl
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cbook as cbook

def read_datafile(file_name):
    # the skiprows keyword is for heading, but I don't know if trailing lines
    # can be specified
    data = np.loadtxt(file_name, delimiter=',', skiprows=10)
    return data

data = read_datafile('e:\dir1\datafile.csv')

x = ???
y = ???

fig = plt.figure()

ax1 = fig.add_subplot(111)

ax1.set_title("Mains power stability")    
ax1.set_xlabel('time')
ax1.set_ylabel('Mains voltage')

ax1.plot(x,y, c='r', label='the data')

leg = ax1.legend()

plt.show()

related stackoverflow.com/questions/11248812/…

Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com

2016-08-29 18:56:08 +00:00
Commented Aug 29, 2016 at 18:56 — Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com, Commented Aug 29, 2016 at 18:56

bmu · Accepted Answer · 2012-11-25 11:32:57Z

48

According to the docs numpy.loadtxt is

a fast reader for simply formatted files. The genfromtxt function provides more sophisticated handling of, e.g., lines with missing values.

so there are only a few options to handle more complicated files. As mentioned numpy.genfromtxt has more options. So as an example you could use

import numpy as np
data = np.genfromtxt('e:\dir1\datafile.csv', delimiter=',', skip_header=10,
                     skip_footer=10, names=['x', 'y', 'z'])

to read the data and assign names to the columns (or read a header line from the file with names=True) and than plot it with

ax1.plot(data['x'], data['y'], color='r', label='the data')

I think numpy is quite well documented now. You can easily inspect the docstrings from within ipython or by using an IDE like spider if you prefer to read them rendered as HTML.

answered Nov 25, 2012 at 11:32

bmu

36.4k13 gold badges93 silver badges108 bronze badges

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

2 Comments

user1850133 Over a year ago

Thanks for your information. I downloaded the numpy ref. manual; it has all the information I need about genfromtxt.

bmu Over a year ago

nice to hear. On Stackoverflow you would normally upvote an answer, if it was useful for you (and maybe leaving a comment) and / or accept it if your question is answered.

Joe Kington · Accepted Answer · 2012-11-25 02:06:09Z

5

I'm guessing

x= data[:,0]
y= data[:,1]

edited Nov 25, 2012 at 2:06

Joe Kington

287k73 gold badges621 silver badges474 bronze badges

answered Nov 24, 2012 at 21:05

user1316487

1 Comment

Joe Kington Over a year ago

Or, alternately, x, y = data[:,:2].T (or even x, y, third_column = data.T if data will always have 3 columns).

Collectives™ on Stack Overflow

plot data from CSV file with matplotlib

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related