Excel makes it easy to add a trendline to charted data.

  1. Right-click the graphed line for actual results. From the menu that appears, choose Add Trendline.
  2. Excel displays the Format Trendline dialog. There are a few settings to change in this dialog. Go to the Trendline Name section. Change the name to Custom. Type a name such as Forecast. Keep the dialog open.
  3. Choose Line Style from the left navigation. Open the Dash Type dropdown and choose a dashed or dotted style. Since this line is only a forecast, you want to differentiate it from the other lines.

The result is a dotted line that shows the predicted results if you continue at your current pace. 

As you continue to plug in actual data, the trendline will redraw.

In other data sets, the chart might only show actuals, with the last actual appearing at the right edge of the chart. In that case, you can use the Format Trendline dialog to specify that the trendline should predict forward a certain number of periods.