[Qgis-developer] Call matplotlib from QGIS

[g_massa at libero.it]

I'm trying to write a simple plugin in Qgis using matplotlib graph. Unfortunately I can't view graph. 
>From a button I've connected this simple function:

import matplotlib.pyplot as plt
[...]
 def BuildGraph(self):
      plt.ion()
      plt.plot([1.6, 2.7])
or:
     plt.show()

I've not response. If I coding in python shell under qgis, I get:
[<matplotlib.lines.Line2D object at 0x03BDFE50>]

I'm setted the matplotlib  beckend with this code  (following:  http://matplotlib.org/faq/usage_faq.html#what-is-a-backend):

import matplotlib
matplotlib.use('Qt4Agg')  

... but the response is the same: nothing!
Could anybody help me? 
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