  We investigate electronic transport through a graphene $n$-$p$ junction in
the quantum Hall effect regime at high perpendicular magnetic field, when the
filling factors in the $n$-doped and $p$-doped regions are fixed to 2 and -2
respectively. We compute numerically the conductance $G$, the noise $Q$ and the
Fano factor $F$ of the junction when inelastic effects are included along the
interface in a phenomenological way, by means of fictitious voltage probes.
Using a scaling approach, we extract the system coherence length $L_\phi$ and
describe the full crossover between the coherent limit ($W\ll L_\phi$) and the
incoherent limit ($W\gg L_\phi$), $W$ being the interface length. While $G$
saturates at the value $e^2/h$ in the incoherent regime, $Q$ and $F$ are found
to vanish exponentially for large length $W$. Corrections due to disorder are
also investigated. Our results are finally compared to available experimental
data.
