My question is why the magnetic field lines goes from plus to minus, if there was two charge. Is it true or isn't it true.
I think if you look carefully at the picture you mentioned, it's showing the electric field, not magnetic. The $E$ label and arrow are the big hit ($E$ is for electric field). The $+$ and $−$ would then be electric charges. Also, $+$ and $−$ are not normally used for magnetic fields, which are always loops. The $N$ and $S$ designations, indicating loops flowing "out" or "in" from bunched-up bundles of such loops, are used instead.