See Three methods for creating Chart Labels that have two lines:
(00:48) Basics of Text Formula and Join Operator, Ampersand.
(01:35) Access Select Data dialog box to edit references in an Excel Chart
(02:07) Text Formula that uses CHAR(10) formula element that represents the ASCII Character Line Return
(03:41) Manual method that requires that we type each Line Return with the Keyboard Alt + Enter
(04:17) Text Formula that manually inserts a Line Return into a formula that can be copied down a column. This is an amazing trick from pmsocho at YouTube