How to always show the plotLine in HighCharts?

In some cases, wergeld's solution would be preferable than jank's solution, especially when you are not sure about min and minRange. But wergeld's solution has a minor issue. If you point your mouse over the plot line, it will show a point and tooltip on the point. To avoid this, I have modified his solution and added enableMouseTracking to get rid of the problem.

{
     name: 'Goal',
     type: 'scatter',
     marker: {
          enabled: false
     },
     data: [450],
     enableMouseTracking: false
}

See updated jsFiddle: http://jsfiddle.net/4R5HH/570/

One other option that does not introduce data points:

yAxis: {
    minRange:450,
    min:0,
    plotLines:[{
        value:450,
        color: '#ff0000',
        width:2,
        zIndex:4,
        label:{text:'goal'}
    }]
},

This sets the minimum for the yAxis to 0 (this is unlikely to be false in this case) and the minimum Range to 450.

See updated fiddle.

You need to add in a point to you chart but disable the marker. I added a new series with scatter plot type and its value equal to the goal value:

{
     name: 'Goal',
     type: 'scatter',
     marker: {
          enabled: false
     },
     data: [450]
}

See updated jsFiddle: http://jsfiddle.net/wergeld/4R5HH/4/