  The phase diagram of the superconducting cuprates is often used to show how
their electronic properties change as a function of the mean doping level,
i.e., the average hole content of the CuO$_2$ plane. In Nuclear Magnetic
Resonance (NMR) experiments average doping, as well as the distribution of
these holes between planar Cu and O reveals itself through the quadrupole
splittings of the $^{63,65}$Cu and $^{17}$O NMR. Here we argue based on all
published NMR data available to us in favor a new type of phase diagram that
has the planar oxygen quadrupole splitting and with it the planar oxygen hole
content as abscissa rather than the average hole content of the CuO$_2$ plane.
In such a plot the superconducting domes of the different cuprate families are
shifted horizontally according to their maximum critical temperature $T_{\rm
c,max}$ set by the chemistry of the parent material, which determines its
oxygen hole content. The higher the O hole content the higher $T_{\rm c,max}$
that can be achieved by actual doping. These findings also offer a strategy for
finding cuprates with higher $T_{\rm c,max}$.
