When plotted on semi-log axes, why is an exponential decay curve not always linear?

 A single-phase exponential decay curve will plot as a straight line when the Y-axis is logarithmic only if the curve plateaus to zero. As implemented in Prism, you would need to constrain the Plateau parameter to a constant value of zero. If the plateau is not 0.0, the exponential decay will look curved, even on with semi-log axes.

Similarly, as shown below (download the Prism file), a two-phase exponential decay curve will appear as two straight lines on a semi-log graph only if the plateau is constrained to zero. If the plateau is not zero, both components will looked curved, even with a logarithmic Y-axis.