error when using Pmw.BLT

[Yang]

I tried to follow the following code demonstrating the use of Pwm.BLT:

I got the following error message after I typed the last line:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Python26\lib\site-packages\Pmw\Pmw_1_3\lib\PmwBlt.py", line
260, in _
_init__
Tkinter.Widget.__init__(self, master, _graphCommand, cnf, kw)
File "C:\Python26\lib\lib-tk\Tkinter.py", line 1932, in __init__
(widgetName, self._w) + extra + self._options(cnf))
_tkinter.TclError: invalid command name "::blt::graph


I am a newbie in python. Does anybody know what causes this error? I
am running python 2.6.3 under windows xp.

Thanks for your help!!