The mechanics of getting a chart to predict future values is easy.
Chart your data in the desired fashion, Right click on the data series,
select "Add Trendline", select the desired trendline, then, under
trendline properties, select forecast however far ahead you want.
Depending on what you want to do with these forecasts, sometimes it can
be better to use worksheet functions (LINEST, TREND, GROWTH, FORECAST,
etc) to predict the data rather than a chart trendline.
The real question, which only you can answer, is how best to forecast
the data. A linear fit of all three points? A linear fit of the last
two points, or the first and last, or the first two points? A
polynomial curve through all three points? Something else? Recognize
that there are many hazards in extrapolating data, especially data like
your sample that don't show a distinct trend.
