Accepted answer

This gray line is called grid line, you can customize them following the reference.

To remove grid lines

 yAxis: {
     gridLineWidth: 0

Like this example.

Change marker width

plotOptions: {
    series: {
        marker: {
            enabled: true,
            symbol: 'circle',
            radius: 7


You can take a look here if you want to customize series markers.

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