To set the attributes for any datum lines:

chSet 'Datum' 'red,dashed,fine'

The mnemonics may be given in any order. The line weights are ‘hair, fine, medium, broad’ or you can give an exact line weight in points. The line styles are ‘solid, dot, dash, ddash, invisible’ or you can use numbers from 1 to 5.

See also ...
Colour Cycle
Line Styles
Nib Weights