Historical clue formulations

• People plot things around it — 2014-06-07
• Bottom line? — 2014-01-31
• Representer of time, often — 2013-06-28
• Plot line — 2013-05-09
• Horizontal graph line — 2013-04-16
• Base line — 2011-01-14
• Horizontal line — 2007-04-01
• Line on which y = 0 — 2006-12-14
• Horizontal line on a graph — 1999-03-01
• The line y = 0, in math — 1997-02-10