Historical clue formulations

• Vertical part of a plane — 2017-05-28
• Vertical line on a graph — 2010-05-17
• Plane part — 2008-03-02
• Graph component — 2006-02-19
• Part of a plane — 2005-06-10
• Vertical line on a graph — 2004-09-06
• Vertical graph component — 2001-07-04