score:1

You defined marker options for area series and you used line series. Correct the options structure and use individual ones per state.

 plotOptions: {
   line: {
     marker: {
       states: {
         hover: {
           fillColor: 'green'
         },
         normal: { ... }
       }
     }
   }
 }

Live demo: http://jsfiddle.net/BlackLabel/er7fnkh0/

API Reference: https://api.highcharts.com/highcharts/plotOptions.line.marker.states.hover


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