Fix setting of closest
Fix setting of closest

Fix the check for setting of closest to an actual hit.  A data series
may have positive length but be entirely out the window.  Fixes #91.

RobertoMalatesta/Flotr2,nicgallardo/Flotr2,HumbleSoftware/Flotr2,beastlike/Flotr2,alanponce/Flotr2,alanponce/Flotr2,nicgallardo/Flotr2,jessiejea/Flotr2,HumbleSoftware/Flotr2,beastlike/Flotr2,beastlike/Flotr2,jessiejea/Flotr2,RobertoMalatesta/Flotr2,alanponce/Flotr2,jessiejea/Flotr2