To set the lower and upper margins:

chSet 'Hmar' 36 12

The margins are specified in points, and should leave room for the heading at the top and the X-axis labels and title at the bottom. If you only give one number, both margins are set to this value. They measure from the edge of the chart area to the edge of the plotting region, i.e. the rectangle bounded by the axes. Please refer to the diagram on the glossary page.

See also ...
Margins – chart margins (tblr)