So currently it's a log function. There's a nice difference between a gap of 1 to 100 years (not too large, not too small) but at the same time the ones in between (take a look at 5 years vs 20 year gaps, the 20 looks like it's only twice the size of the 5 year) may not necessarily be the size you'd like them to be, so play around with it and tell me if you'd like some specific differences, i.e. you'd like gaps closer to min to grow slower or gaps closer to max to grow faster. An alternative function I'd suggest if you don't like the log is the sigmoid curve (https://en.wikipedia.org/wiki/Logistic_function)
So currently it's a log function. There's a nice difference between a gap of 1 to 100 years (not too large, not too small) but at the same time the ones in between (take a look at 5 years vs 20 year gaps, the 20 looks like it's only twice the size of the 5 year) may not necessarily be the size you'd like them to be, so play around with it and tell me if you'd like some specific differences, i.e. you'd like gaps closer to min to grow slower or gaps closer to max to grow faster. An alternative function I'd suggest if you don't like the log is the sigmoid curve (https://en.wikipedia.org/wiki/Logistic_function)