gnuplot style index plotting using numpy and matplotlib

Question

I have been using gnuplot for as long as I remember. Recently i have been wanting to switch to matplotlib.

One basic function of gnuplot i frequently use is the 'index' function.

Suppose I have the following data file (foo.dat):

I could do:

plot 'foo.dat' index 0 u 1:2
replot 'foo.dat' index 1 u 1:2

to get two lines corresponding to the two data sets in the same plot. How would one import such a file using numpy and plot it using matplotlib?

Here is how I would like my final plot to look like:

Seems like there is no direct way to mimic gnuplot'a block indexing with some options. See Reading data blocks from a file in Python for a very similar question. — Christoph
– Christoph, Commented Nov 4, 2015 at 19:12

Lee · Accepted Answer · 2015-11-05 15:02:35Z

3

Use genfromtxt to read in the file:

import numpy as np
import matplotlib.pyplot as plt

a = np.genfromtxt('foo.dat')

gives:

array([[ 1.,  1.],
       [ 2.,  2.],
       [ 3.,  3.],
       [ 1.,  1.],
       [ 2.,  4.],
       [ 3.,  9.]])

Then plot:

plt.plot(a[3:,0],a[3:,1],marker='x',color='g')
plt.plot(a[0:3,0],a[0:3,1],marker='x',color='r')

gives:

EDIT

Following @Cristoph's comment

To read in automatically, you could split the file into chunks, if you read in with pandas. But you'd need to write a script to find the rows to skip for a large file.

import pandas as pd
b=pd.read_csv('foo.dat',sep=' ',chunksize=3,header=None,skiprows=(3,4),index_col=0)
for c in b:
    plt.plot(c)

Note the skiprows parameter - you'd need to create a list to fit your file if it's longer.

edited Nov 5, 2015 at 15:02

answered Nov 4, 2015 at 9:41

Lee

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4 Comments

PyariBilli Over a year ago

This is not exactly what I want. I have edited my question to add a figure of what i want.

Christoph Over a year ago

The important point is, that gnuplot uses the two empty lines to automatically recognize different blocks, which can then be accessed with index.

Christoph Over a year ago

That's not automatic, you must still give concrete numbers. You cannot say: plot the first two blocks, no matter how many points each contains, which is what gnuplot does...

Lee Over a year ago

@Christoph In that case the long answer to gforce89's comment is "yes, but you need to write a script to find the spaces between blocks."

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