  As an alternative to the popular parametrisations of the dark energy equation
of state, we construct a quintessence model where the scalar field has a linear
dependence on the number of e-folds. Constraints on more complex models are
typically limited by the degeneracies that increase with the number of
parameters. The proposed parametrisation conveniently constrains the evolution
of the dark energy equation of state as it allows for a wide variety of time
evolutions. We also consider a non-minimal coupling to cold dark matter. We fit
the model with Planck and KiDS observational data. The CMB favours a
non-vanishing coupling with energy transfer from dark energy to dark matter.
Conversely, gravitational weak lensing measurements slightly favour energy
transfer from dark matter to dark energy, with a substantial departure of the
dark energy equation of state from -1.
