score:1

Accepted answer
  1. use plot lines for y-axis:

    yaxis: {
        ...
        plotlines: [{
            color: 'red',
            value: 100,
            zindex: 2
        }]
    }
    
  2. use gridlinecolor for x-axis:

    xaxis: {
        ...,
        gridlinecolor: 'blue'
    }
    
  3. use gridlinecolor for y-axis:

    yaxis: {
        gridlinecolor: 'green',
        ...
    }
    

live demo: http://jsfiddle.net/blacklabel/fm45ab3k/

api reference: https://api.highcharts.com/highcharts/yaxis.plotlines


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