Add a Vertical Datum Line
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From the Line, Bar, Complex, Polar, Vector, or Radar Chart ribbons, Datums tool
group, click the Add Vertical Datum Lines tool.
Figure 1. 
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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.