The X-axis caption is written below the end of the axis, spaced to allow room for labels (if any):

chSet 'xcap' '1994'

You can control the positioning with the X-axis style to place the caption centred (below the axis) or at the right-hand end. The caption text will be split over more than one line of you include a semi-colon in the text, for example:

chSet 'xcap' 'Elapsed days;from start of project'

To get a semi-colon into a caption, preceed it with a backslash character.

See also ...
CaptionFont – font for all axis captions
CaptionStyle – caption style
XCaptionFont – font for X caption
XStyle – X-axis style