I ended up with ebook(t)=k * (1 + tanh(t)) Where k is a constant one selects out of one???s ass (a surprising number of scientific constants work that way) and t is the time variable. t, in my graph above, is counted off in units of [roughly] ??/60, and t=-?? at some arbitrary point in the past ???in this case, November of 2006: one year before the kindle came out.