[toast76]

The difficulty possibly stems from starting at 0 and trying to get a growth function to suit over 1-5 years (ideally it's exponential in the early days, but likely logarithmic over a longer period of time). Problem is if you're going to investors, you don't want a chart that shows growth slowing down over time (certainly not in the first 24 months anyway).

Early on you have an idea of what sort of traffic you're able to generate and a likely conversion rate, and therefore estimate a sign up rate using real(ish) numbers. It's only the growth beyond that that you're really guessing at.

You may just want a text box with a gross # of new customers/month based on what you think you can bring in, and then have the slider work on top of that, in effect creating an adjustable polynomial function for growth? Just spitballing here...