Most things in HighCharts have smart defaults, so unset options will do something reasonable. But plotLines is not this way: if the user does not explicitly specify a width and color, no line will be drawn (and sometimes a JS error will happen).

Example which works but includes comments on how to easily break it: http://jsfiddle.net/LKayp/

I think plotLines (and addPlotLines) should default to a reasonable width and color, just as regular series lines do.