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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