In such an unrestricted situation, simply use polyfit and its friends polyval and others (all linked to at the end of the page).
I suggest you start with a line (degree 1) and increase the order of the polynomial fit to get one that makes sense to you but does not ‘overfit’ your data.
If you want to minimise the error, calculate the sum-of-squares of the residuals with each increase in order. (You can actually put this in a loop if you like.) When they’re small enough and you’re happy with the way it looks, stop. Something less than a 5th degree polynomial will likely be optimal.
