Trendline

In Excel, a trendline shows the slope of data points. They can be added to any data series in an
unstacked, 2-D, area, bar, column, line, stock, xy (scatter), or bubble chart. It is always associated with a data series, but a trendline does not represent the data of that data series.
Instead, it is used to depict trends in your existing data or forecasts of future data.