  Rare-earth titanates are Mott insulators whose magnetic ground state --
antiferromagnetic (AFM) or ferromagnetic (FM) -- can be tuned by the radius of
the rare-earth element. Here, we combine phenomenology and first-principles
calculations to shed light on the generic magnetic phase diagram of a
chemically substituted titanate on the rare-earth site that interpolates
between an AFM and a FM state. Octahedral rotations present in these
perovskites cause the AFM order to acquire a small FM component -- and
vice-versa -- removing any multicritical point from the phase diagram. However,
for a wide parameter range, a first-order metamagnetic transition line
terminating at a critical endpoint survives inside the magnetically ordered
phase. Like the liquid-gas transition, a Widom line emerges from the endpoint,
characterized by enhanced fluctuations. In contrast to metallic FMs, this
metamagnetic transition involves two symmetry-equivalent and insulating canted
spin states. Moreover, instead of a magnetic field, we show that uniaxial
strain can be used to tune this transition to zero temperature, inducing a
quantum critical endpoint.
