This chart shows actuals so far in a solid line with forecast numbers in a dashed line.

Worksheet data used for line chart

Worksheet data used for line chart

Worksheet data used for line chart

Worksheet data used for line chart

Worksheet data used for line chart

Insert line chart from ribbon

Select first option for basic line chart

After inserting a basic line chart

Set dashed line under Format Series

After setting dashed line on 2nd data series