To set the attributes for both X and Y grid lines:

chSet 'Grid' 'green,dotted,fine'

The mnemonics may be given in any order. The line styles are ‘solid, dot, dash, ddash, invisible’ or you can use numbers from 1 to 5 (or higher numbers to use a custom dash pattern).

See also ...
Colour Cycle
Line Styles
Nib Weights