Years to double portfolio

I wondered how long it would take me to double my portfolio value. I started to play with excel and the formula.

Let t=time to double and r=annual return (in percentage)

t=Ln 2 / Ln (1+r)

As I played with the formula, I decided to practice some risk management and created a sensitivity graph of the annual percentage range and the number of years it would take to double a portfolio.

Here is a graph: