How to read the convergence rate from a plot?

Hello, I have a convergence plot and i know that how to check the convergence mathematically and how to plot it. but if i have a plot only and i want to guess the convergence rate then what is the method for that? consider the following plots: In plot 1 i have inserted slop and it is clear that what is rate of convergence. Let we have plot 2 then how to find convergence rate by reading graph only?

Thanks in advance

Bibigul