Possible Clues:

• Going along the line, briefly?
• Lacking depth, briefly
• Like the x-, y- or z-axis
• Linear, informally
• Like straight lines, for short
• Lacking height and depth, for short
• Group with the 2012 chart-topping album 'Up All Night,' to fans
• Like a straight line, in brief
• Like the number line, in brief
• Breakout band for Harry Styles and Zayn Malik, familiarly