我尝试在centos 7机器上编译matplotlib 1.4.2和pylab，并尝试运行以下程序：
#!/bin/python
import numpy as np
import matplotlib
import pylab as pl
X = np.linspace(-np.pi, np.pi,256,endpoint=True)
C = np.cos(X)
pl.plot(X,C)
当我运行此程序时，我收到以下错误消息：
Traceback (most recent call last):
File "check.py", line 11, in <module>
pl.plot(X,C)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux- x86_64.egg/matplotlib/pyplot.py", line 3092, in plot
ax = gca()
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux- x86_64.egg/matplotlib/pyplot.py", line 828, in gca
ax = gcf().gca(**kwargs)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux- x86_64.egg/matplotlib/pyplot.py", line 462, in gcf
return figure()
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux-x86_64.egg/matplotlib/pyplot.py", line 435, in figure
**kwargs)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux- x86_64.egg/matplotlib/backends/backend_qt4agg.py", line 47, in new_figure_manager
return new_figure_manager_given_figure(num, thisFig)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt4agg.py", line 54, in new_figure_manager_given_figure
canvas = FigureCanvasQTAgg(figure)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux- x86_64.egg/matplotlib/backends/backend_qt4agg.py", line 72, in __init__
FigureCanvasQT.__init__(self, figure)
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt4.py", line 68, in __init__
_create_qApp()
File "/usr/lib64/python2.7/site-packages/matplotlib-1.4.2-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5.py", line 138, in _create_qApp
raise RuntimeError('Invalid DISPLAY variable')
RuntimeError: Invalid DISPLAY variable
知道可能导致这些错误的原因吗？如何用matplot lib 1.4.2生成图？我确实回显了变量$ DISPLAY，没有出现任何内容。我该如何解决这个问题？
感谢您的时间。