I have monthly data for these prices from 2005-2018. However, I want to have an approximation for the next 5 years. Using TREND() function at excel worked for CPI as it has a "linear" trend but when it comes to GNL, Diesel or Brent I don't want to include in my study just a linear approximation (although I can assume that this price maintains overtime).
I don't have a great statistics background and for my purposes I need a simple approximation, if it's possible. Right now, I' reading about ARIMA that could be of great use.