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 positive sound speed are excluded at the 95 % C. L .