Anyone knows how to derive a function for this chat?
y=f(x)=???
Thanks for your attention.
This is of the general form:
$\displaystyle y=m \log(x) + c$
To find $\displaystyle m$ and $\displaystyle c$ from the graph it is probably best to use logarithms to the base $\displaystyle 10$, so:
$\displaystyle y=m \log_{10}(x) + c$
Now you can put $\displaystyle u=\log_{10}(x)$, and relabel the horizontal axis with the corresponding values of $\displaystyle u$, and you will have a standard straight line plot:
$\displaystyle y=m u +c $
CB