  We define a quasar-galaxy mixing diagram using the slopes of their spectral
energy distributions (SEDs) from 1\mu m to 3000\AA\ and from 1\mu m to 3\mu m
in the rest frame. The mixing diagram can easily distinguish among
quasar-dominated, galaxy-dominated and reddening-dominated SED shapes. By
studying the position of the 413 XMM selected Type 1 AGN in the wide-field
"Cosmic Evolution Survey" (COSMOS) in the mixing diagram, we find that a
combination of the Elvis et al. (1994, hereafter E94) quasar SED with various
contributions from galaxy emission and some dust reddening is remarkably
effective in describing the SED shape from 0.3-3\mu m for large ranges of
redshift, luminosity, black hole mass and Eddington ratio of type 1 AGN. In
particular, the location in the mixing diagram of the highest luminosity AGN is
very close (within 1\sigma) to that of the E94 SED. The mixing diagram can also
be used to estimate the host galaxy fraction and reddening in quasar. We also
show examples of some outliers which might be AGN in different evolutionary
stages compared to the majority of AGN in the quasar-host galaxy co-evolution
cycle.
