Assume your file looks like this and is named test.txt (space delimited):
1 2
3 4
5 6
7 8
Then:
#!/usr/bin/python
import numpy as np
import matplotlib.pyplot as plt
with open("test.txt") as f:
data = f.read()
data = data.split('\n')
x = [row.split(' ')[0] for row in data]
y = [row.split(' ')[1] for row in data]
fig = plt.figure()
ax1 = fig.add_subplot(111)
ax1.set_title("Plot title...")
ax1.set_xlabel('your x label..')
ax1.set_ylabel('your y label...')
ax1.plot(x,y, c='r', label='the data')
leg = ax1.legend()
plt.show()
I find that browsing the gallery of plots on the matplotlib site helpful for figuring out legends and axes labels.