  Using cosmological galaxy simulations from the MaGICC project, we study the
evolution of the stellar masses, star formation rates and gas phase abundances
of star forming galaxies. We derive the stellar masses and star formation rates
using observational relations based on spectral energy distributions by
applying the new radiative transfer code GRASIL-3D to our simulated galaxies.
The simulations match well the evolution of the stellar mass-halo mass
relation, have a star forming main sequence that maintains a constant slope out
to redshift z $\sim$ 2, and populate projections of the stellar mass - star
formation - metallicity plane, similar to observed star forming disc galaxies.
We discuss small differences between these projections in observational data
and in simulations, and the possible causes for the discrepancies. The
light-weighted stellar masses are in good agreement with the simulation values,
the differences between the two varying between 0.06 dex and 0.20 dex. We also
find a good agreement between the star formation rate tracer and the true
(time-averaged) simulation star formation rates. Regardless if we use mass- or
light-weighted quantities, our simulations indicate that bursty star formation
cycles can account for the scatter in the star forming main sequence.
