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