How do I get the secondary axis to cross at zero

[sparky]

Using an XY scatter adding a secondary axis does not line up to zero. Very
difficult to compare two graphs with different zero points!!

 

[Debra Dalgleish]

Jon Peltier has instructions on his web site:

Align X Axis to Y=0 on Two Y Axes
http://peltiertech.com/Excel/Charts/AlignXon2Ys.html