Lets make my scenario not generic just so that i could use particular terms
Say i have a graph of population per year of someplace over some decades
- How can i calculate expected\estimated population next year and son on and so forth
The link doesn't work for me, but your first stop should be linear regression. If you take the logarithm of the population you will be looking for an exponential increase-a fixed percentage change per year instead of a fixed number change per year. This is probably a better fit. Any numerical analysis text will have a chapter on this, or you can get Excel to do it. Graph the data as a scatter plot, right click on the graph, choose Add Trendline, and pick the kind of trendline you want.