  Protoplanetary discs are the site of star and planet formation, and their
evolution and consequent dispersal deeply affect the formation of planetary
systems. In the standard scenario they evolve on timescales ~Myr due to the
viscous transport of angular momentum. The analytical self-similar solution for
their evolution predicts also specific disc isochrones in the accretion rate -
disc mass plane. However, photoevaporation by radiation emitted by the central
star is likely to dominate the gas disc dispersal of the innermost region,
introducing another (shorter) timescale for this process. In this paper, we
include the effect of internal (X and EUV) photoevaporation on the disc
evolution, finding numerical solutions for a population of protoplanetary
discs. Our models naturally reproduce the expected quick dispersal of the inner
region of discs when their accretion rates match the rate of photoevaporative
mass loss, in line with previous studies. We find that photoevaporation
preferentially removes the lightest discs in the sample. The net result is
that, counter-intuitively, photoevaporation increases the average disc mass in
the sample, by dispersing the lightest discs. At the same time,
photoevaporation also reduces the mass accretion rate by cutting the supply of
material from the outer to the inner disc. In a purely viscous framework, this
would be interpreted as the result of a longer viscous evolution, leading to an
overestimate of the disc age. Our results thus show that photoevaporation is a
necessary ingredient to include when interpreting observations of large disc
samples with measured mass accretion rates and disc masses. Photoevaporation
leaves a characteristic imprint on the shape of the isochrone. Accurate data in
the accretion rate - disc mass plane in the low disc mass region therefore give
clues on the typical photoevaporation rate.
