To set the right margin:

chSet 'MRight'  48 

This allows space (in points) from the end of the X-axis to the edge of the chart region. It is set by default to a small value, so you will need to increase it (say to 48 points) if you have a duplicate or right-hand Y-axis.

See also ...
Margins – chart margins (tblr)
Glossary of Terms