There is some evidence that linear hair growth rate, meaning how fast your beard grows, is determined by the amount of dihydrotestosterone (DHT) you produce. DHT is a byproduct of testosterone, activated by an enzyme in the hair follicle's oil glands. Beard growth rates can vary considerably.

Does hair grow at a linear rate?

Fitting a linear function to the other straight parts gives a very similar slope. So, I guess for this guy hair grows around 0.03 centimeter a day. With this rate, if he didn't cut his hair for 8 years (almost 3,000 days) his hair would grow about 0.9 meters (assuming a linear growth rate).

Is hair growth linear or exponential?

The numbers of hairs in each length state follow an exponential decay curve as the length of the state increases. This follows from what I had already shown regarding the probability of each length state earlier. For simplicity, let q be the complement of p: q=1-p.