Excel Custom trendlines and R^2 values

[JesterPear3737]

I have been struggling with a problem for the last hour or so, and as someone who does not normally use Excel I am getting nowhere.

I have been plotting population data from the world's most populous countries over the past 50 years, attempting to find if the population growth conforms to an exponential model enough to make accurate predictions about future populations (long story short, no you can't model population growth to any respectable degree of accuracy with an exponential function).

I've found using a semi-log plot the most accurate equations for the datasets, but I would like to display the model on a scatterplot showing the data I have already.found, and have Excel calculate an R^2 value for that model.

I don't know if this is possible to do, but I would appreciate any help with this project.

Thanks.