  Universal scaling laws can guide the understanding of new phenomena, and for
cuprate high-temperature superconductivity such an early influential relation
showed that the critical temperature of superconductivity ($T_c$) correlates
with the density of the superfluid measured at low temperatures. This famous
Uemura relation has been inspiring the community ever since. Here we show that
the charge content of the bonding orbitals of copper and oxygen in the
ubiquitous CuO$_2$ plane, accessible with nuclear magnetic resonance (NMR), is
tied to the Uemura scaling. This charge distribution between copper and oxygen
varies between cuprate families and with doping, and it allows us to draw a new
phase diagram that has different families sorted with respect to their maximum
$T_c$. Moreover, it also shows that $T_c$ could be raised substantially if we
were able to synthesize materials in which more oxygen charge is transferred to
the approximately half filled copper orbital.
