  We classify dark energy models in a plane of observables that correspond to
the common parameterization of a non-constant equation of state, w(a)=w_0 +
w_a(1-a), where $a$ is the scale factor of the universe. The models fall into
four classes and only two of these classes have a region of overlap in the
observable plane. We perform a joint analysis of all Type Ia supernova (SNIa)
data compiled by the High-Z SN Search Team (HZT) and the Supernova Legacy
Survey (SNLS) and find that no class of models is excluded by current SNIa
data. However, an analysis of large scale structure, Ly$\alpha$ forest and bias
constraints from SDSS, the Gold SNIa data and WMAP data indicates that
non-phantom barotropic models with a a positive sound speed are excluded at the
95% C.L.

High Energy Physics - Phenomenology;Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory