why matplotlib give the error []?
Had this problem. You just have to use show() function to show it in a window. Use pyplot.show()
I am using python 2.7.9 on win8. When I tried to plot using matplotlib, the following error showed up:

from pylab import *
  plot([1,2,3,4])
[matplotlib.lines.Line2D object at 0x0392A9D0]

I tried the test code "python simple_plot.py --verbose-helpful", and the following warning showed up:

$HOME=C:\Users\XX
  matplotlib data path C:\Python27\lib\site-packages\matplotlib\mpl-data

You have the following UNSUPPORTED LaTeX preamble customizations:
Please do not ask for support with these customizations active.

loaded rc file C:\Python27\lib\site-packages\matplotlib\mpl-data\matplotlibrc
  matplotlib version 1.4.3
  verbose.level helpful
  interactive is False
  platform is win32
  CACHEDIR=C:\Users\XX.matplotlib
  Using fontManager instance from C:\Users\XX.matplotlib\fontList.cache
  backend TkAgg version 8.5
  findfont: Matching :family=sans-serif:style=normal:variant=normal:weight=normal:stretch=normal:size=medium to Bitstream
  Vera Sans (u'C:\Python27\lib\site-packages\matplotlib\mpl-data\fonts\ttf\Vera.ttf') with score of 0.000000

What does this mean? How could I get matplotlib working?