Werner,
Any chance you just need to issue the call to pyplot as matplotlib.pyplot?
Those entries in your Desktop10.pth file look OK.
Your %PATH% variable should have a C:\Python26\ArcGIS10.0 entry, and python.exe and pythonw.exe should only be in that directory.
Also, have a look in the registry at this Key:
HKEY_LOCAL_MACHINE\SOFTWARE\Python\PythonCore\2.6\PythonPath
Note: would be SOFTWARE\Wow6432Node\Python on 64-bit OS.
The key's Default value should have a lib-tk value laid down by the matplotlib installer that matches ESRI's Python 2.6 instance. Something like this:
[INDENT]C:\Python26\ArcGIS10.0\Lib;C:\Python26\ArcGIS10.0\DLLs;C:\Python26\ArcGIS10.0\Lib\lib-tk
[/INDENT]
If it was not laid down in the same path, you'll need to uninstall matplotlib, and numpy packages. Then reinstall numpy-1.3.0 and matplotlib-1.0.1.
When the package installer GUIs run be sure to choose the C:\Python26\ArcGIS10.0 instance of Python.
Stuart