Syntax:
set terminal imagen {<fontsize>} {portrait | landscape} {[<horiz>,<vert>]}
where fontsize defaults to 12 points and the layout defaults to landscape. 5#5horiz6#6 and 5#5vert6#6 are the number of graphs in the horizontal and vertical directions; these default to unity.
Example:
set terminal imagen portrait [2,3]
puts six graphs on the page in three rows of two in portrait orientation.