Not sure if you were using the old bar chart or the new line chart, but I believe the algorithm was the same as follows:

• Best Guess: Projects using average of last 3 iteration velocities
• Optimistic: Projects using average of best 3 from last 6 iteration velocities
• Pessimistic: Projects using average of worst 3 from last 6 iteration velocities

I did go verify, but I think you actually need to include the required history data (i.e. 3 or 6 iterations) in the chart or it won't do the forecasts.  I would prefer that it could go get the data for the purposes of forecasting, but not have to include them in the chart (because often those iterations aren't part of the current "release").