  A fundamental issue concerning iron-based superconductivity is the roles of
electronic nematicity and magnetism in realising high transition temperature
($T_{\rm c}$). To address this issue, FeSe is a key material, as it exhibits a
unique pressure phase diagram involving nonmagnetic nematic and
pressure-induced antiferromagnetic ordered phases. However, as these two phases
in FeSe overlap with each other, the effects of two orders on superconductivity
remain perplexing. Here we construct the three-dimensional electronic phase
diagram, temperature ($T$) against pressure ($P$) and isovalent S-substitution
($x$), for FeSe$_{1-x}$S$_{x}$, in which we achieve a complete separation of
nematic and antiferromagnetic phases. In between, an extended nonmagnetic
tetragonal phase emerges, where we find a striking enhancement of $T_{\rm c}$.
The completed phase diagram uncovers two superconducting domes with similarly
high $T_{\rm c}$ on both ends of the dome-shaped antiferromagnetic phase. The
$T_{\rm c}(P,x)$ variation implies that nematic fluctuations unless
accompanying magnetism are not relevant for high-$T_{\rm c}$ superconductivity
in this system.
