  We study the effect of local modulation of charge density and carrier type in
graphene field effect transistors using a double top gate geometry. The two top
gates lead to the formation of multiple \emph{p-n} junctions. Electron
transport measurements at low temperature and in the presence of magnetic field
show various integer and fractionally quantized conductance plateaus. We
explain these results based on the mixing of the edge channels and find that
inhomogeneity plays an important role in defining the exact quantization of
these plateaus, an issue critical for the metrology applications of \emph{p-n}
junctions.
