High and low plot lines
allow for a low and high plot line on a graph so if the line goes above or below these plot lines the colour of the graph line changes. A nice instant way of visualising if a graph goes beyond certain 'ideal' limits.
This was completed with series.zones (http://api.highcharts.com/highcharts/plotOptions.series.zones), which was introduced in 4.1.0
Roy Tinker commented
This should be merged with the "Threshold" feature.