To set the top margin:

chSet 'Mtop'  36 

This allows space (in points) for the chart heading and sub-heading. You will probably need to increase it if you have a heading that spans more than 2 lines. If you use left or right aligned headings for Pie or Polar charts, you can probably reduce this to zero as the headings will not clash with the chart in this case.

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