In my AWS Ubuntu 14.02 instance I installed the matplotlib dependencies and then via pip I installed matplotlib:

sudo apt-get build-dep python-matplotlib

sudo pip install matplotlib

sudo pip freeze:
matplotlib==1.4.0

in a brand new file plotting1.py I wrote:

!/usr/bin/python

from pylab import *
plot([1,2,3])
show()

after typying chmod +x plotting1.py, I executed the little script:

time python plotting1.py
Traceback (most recent call last):
File "plotting1.py", line 4, in
plot([1,2,3])
File "/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 3086, in plot
ax = gca()
File "/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 827, in gca
ax = gcf().gca(*_kwargs)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 461, in gcf
return figure()
File "/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 434, in figure
*_kwargs)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt4ag g.py", line 47, in new_figure_manager
return new_figure_manager_given_figure(num, thisFig)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt4ag g.py", line 54, in new_figure_manager_given_figure
canvas = FigureCanvasQTAgg(figure)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt4ag g.py", line 72, in init
FigureCanvasQT.init(self, figure)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt4.p y", line 68, in init
_create_qApp()
File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt5.p y", line 139, in _create_qApp
raise RuntimeError('Invalid DISPLAY variable')
RuntimeError: Invalid DISPLAY variable

real 0m0.404s
user 0m0.284s

What do I have to do now?

Looking forward to your kind help.
Kind regards.
Marco