Possible bug in axis labelling

[<al...@ac...>]

For 7 plots, two of the plots have -1.38778e-17 instead of 0. Though the
zero value isn't always -1.38778e-17.  You can find a screenshot of
incorrect and correct axis here:
http://accipter.org/pub/qwt-scale-problem.png

I have tried my program in both linux and windows environments, and it
appears to only happen on windows (qt-4.4.0 with mingw).

Let me know if you need me to supply the code.

Albert