Dear Scabral,
The line you see is the horizontal axis which is placed automatically. You can solve this or control this by adding the option AxesOrigin -> {x,y}.
Maarten
Thank you so much, Maarten ! It does work ! Dear JIm, thank you, too! Fortunatley, it is not a bug ! Scabral
I wish I could supply a definite answer but I used to get such lines (usually unexpected vertical lines) when using Mathematica version 9 but the same code does not generate those lines in version 10 (running on Windows 7).
