The main chart heading is set with:

chSet 'Heading' 'Here is;the Main Heading'

It is centred by default above the chart. New lines are forced with a semi-colon in the text (to get a semi-colon, preceed it with a backslash character). You can also make a second heading line in a different font by specifying a subhead:

chSet 'Sub' 'This is a subordinate heading'

Heading and subhead share the same placement, but you can set the subhead font independently.

See also ...
HeadingFont – heading font
HeadingStyle – heading placement