Add a Horizontal Datum Line
- 
                From the Line, Bar, Complex, Polar, Vector, or Radar Chart ribbons, Datums tool
                    group, click the Add Horizontal Datum Lines tool.
                

Figure 1. - 
                Select the point on the curve to add the datum line to and left-click.
                The datum line is added to the curve and labeled "Datum 'n'".
 - Click Esc to exit the datum tool and perform other functions.