How to find doubling time of cells with scatterplot in R?

I'm trying to calculate the doubling time of cells using a scatterplot. This is my dataframe

df = data.frame("x" = 1:5, "y" = c(246, 667, 1715, 4867, 11694))

and I've graphed this dataframe using this code

plot(x, df$y, xlab = "days", ylab = "cells mL -1")

Does anyone know how to calculate the doubling time of these cells using the graph? the equation for doubling time is (ln(2)/rate constant)