  We constrain an interacting, holographic dark energy model, first proposed by
two of us in [1], with observational data from supernovae, CMB shift, baryon
acoustic oscillations, x-rays, and the Hubble rate. The growth function for
this model is also studied. The model fits the data reasonably well but still
the conventional $\Lambda$CDM model fares better. Nevertheless, the holographic
model greatly alleviates the coincidence problem and shows compatibility at
$1\sigma$ confidence level with the age of the old quasar APM 08279+5255.
