[Solved]-Highcharts - Highlight / Shade date range-Highcharts

Unless you really need the legend entry, I would recommend using plotBands instead

http://api.highcharts.com/highcharts#xAxis.plotBands

You can also do it like this, if you do really need the legend:

http://jsfiddle.net/jlbriggs/JVNjs/305/

data:[[1.5,0],[1.5,80],[2.25,80],[2.25,0]]

It relies in part on setting a min and max, and using those min and max values as your y data points.