you can do this by adding a plotline (api). plot lines can be across the x-axis or y-axis, and in your case you need a line along the y-axis:

yaxis: {
    plotlines: [{
        color: 'red',
        width: 2,
        value: 120,
        zindex: 5

as you can see you can set color, width and the value the line will be at. here zindex is used to make the line come in front of the columns.

see this jsfiddle demonstration of it in use.

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