Bi-exponential regression, (two-phase decay)
Fits the model defined by
$SpanFast=(Y0-Plateau)\cdot PercentFast\cdot 0.01$
$SpanSlow=(Y0-Plateau)\cdot (100-PercentFast)\cdot 0.01$
$Y=Plateau + SpanFast\cdot\exp(-KFast\cdot X) + SpanSlow\cdot\exp(-KSlow\cdot X)$