  Broadband flux measurements centred around [3.6 $\mu$m] and [4.5 $\mu$m]
obtained with Spitzer during the occultation of seven extrasolar planets by
their host stars have been combined with parallax measurements to compute the
absolute magnitudes of these planets. Those measurements are arranged in two
colour-magnitude diagrams. Because most of the targets have sizes and
temperatures similar to brown dwarfs, they can be compared to one another. In
principle, this should permit inferences about exo-atmospheres based on
knowledge acquired by decades of observations of field brown dwarfs and
ultra-cool stars' atmospheres. Such diagrams can assemble all measurements
gathered so far and will provide help in the preparation of new observational
programs. In most cases, planets and brown dwarfs follow similar sequences.
HD\,2094589b and GJ 436b are found to be outliers, so is the nightside of HD
189733b. The photometric variability associated with the orbital phase of HD
189733b is particularly revealing. The planet exhibits what appears like a
spectral type and chemical transition between its day and night sides: HD
189733b straddles the L-T spectral class transition, which would imply
different cloud coverage on each hemisphere. Methane absorption could be absent
at its hot spot but present over the rest of the planet.
