# This line is only needed if you're using a jupyter notebook 
# if you don't know what that is, you're probably not using one :) 
# don't include it if you're only writing a script
%matplotlib inline

# This imports all the libraries we need
import matplotlib
import pylab as pl
import numpy as np

Let's create a file called mydata.txt

filename = "mydata.txt"
f = open(filename, "w")

Let's create two lists of numbers we'll write to it: x and y

x = [0, 1, 2, 3, 4, 5, 6]
y = [0, 1, 4, 9, 16, 25, 36]

writing to the file

# for every value in x and y we need to write a line
# python needs us to convert the number to a string first (str), and then add a newline character at the end 
for (xvalue, yvalue) in zip(x,y):
    f.write(str(xvalue) + " " + str(yvalue) + "\n")
        
# close the file after writing    
f.close()

Now, let's open it

data = np.genfromtxt("mydata.txt")
print(data)

[[  0.   0.]
 [  1.   1.]
 [  2.   4.]
 [  3.   9.]
 [  4.  16.]
 [  5.  25.]
 [  6.  36.]]

Let's assign names to the first and second columns

xx = data[:,0]
yy = data[:,1]

print(xx)

[ 0.  1.  2.  3.  4.  5.  6.]

print(yy)

[  0.   1.   4.   9.  16.  25.  36.]

Now, let's plot it

pl.plot(xx,yy)

[<matplotlib.lines.Line2D at 0x1105b8ef0>]

You can also plot it as a scatter plot

pl.scatter(xx,yy)

<matplotlib.collections.PathCollection at 0x1106b4da0>

Let's create a figure, so that we can control how it's plotted

fig = pl.figure() # Create figure
ax = fig.add_subplot(111) # add some axes to it



# Add some labels

ax.set_xlabel("Oranges")
ax.set_ylabel("Apples") 

# Tweak the color and size of the scatter points

ax.scatter(xx, yy, color='red', s=100)

# change the axes limits
ax.set_xlim(-2,9)

(-2, 9)

Save the plot as an image

fig.savefig("mydata.png")