Isochron

Recapitulating, any suite of comagmatic rocks with an age t defines an isochron if its members all had the same initial ^{₈₇}Sr/^{₈₆}Sr ratio and if it was closed to both Rb and Sr since crystallization occurred and neither element was introduced from any extraneous source.[…]In this way, whole-rock Rb-Sr isochrons can produce rather reliable age data for rock samples.

The entire plane is foliated by isochrons.