procedure SetLabelText(dataName, whichLabel, labelText, fontName : Str255; fontSize : integer; style : Str255);

Sets the label specified by whichLabel (XLABEL, YLABEL, ZLABEL, TITLE).  The label text, font, font size and face can be set.  The style is specified by a string like:  'BOLD+ITALIC' for bold italic.

Also see:  SetLabelColor  SetAxisText