  We characterize the heterogeneous character of the dynamics of liquids
approaching the glass transition through an experimental determination of the
number of dynamically correlated molecules N_{corr} as obtained from dynamical
susceptibilities. To do so, we have obtained a new set of dielectric
spectroscopy data for liquid dibutyl-phtalate on a fine and extended
temperature and pressure grid, and we have used it in conjunction with
high-pressure data from the literature. We have been able to evaluate the
contributions to N_{corr} that are due to fluctuations associated with density
and with temperature separately, thereby improving the estimate of N_{corr}. We
find that N_{corr} increases along the glass transition line, and more
generally along any isochronic line, as pressure increases (up to 1 GPa), a
result which is at odds with recent reports and theoretical predictions.
