The following options specify a key.
key - the legend.
keyfont - the font.
keypos - a list of up to three positions from:
First-letter abbreviations are supported. The default is lti, or equivalently:
left top inside
keystyle - a list of up to four styles from:
First-letter abbreviations are supported. The default is lbvt, or equivalently:
left boxed vertical thin
Example
pd 'key Canada China India Japan USA' pd 'keyfont Arial 8' pd 'keypos boxed right' pd 'keystyle for'