Question

I already have a basic line graph for highcharts. What I wanna do is add just a straight line across (with a constant y = some value) straight through the range of the x’s.

Is there a way to do that or do I have to put data points everywhere even if its the same y value everytime?

Answer 1

What you are looking for is called a plotLine.
This will let you specify a yValue and it just goes straight across regardless of zoom level on xAxis.
You would add it like this:

yAxis: {
  plotLines: [{
    color: 'blue',
    width: 2,
    value: 150,
    dashStyle: 'longdashdot'               
  }]
}