HighCharts: How to change style of bottom line color

enter image description here I want to change the color of the line to black one as you can see above

this is my code of that part which draws highcharts.

enter image description here I thought I needed to fix it in the type:'area' and I've tried to set the type area and read API as much as I can but I couldn't solove the problem

plz anybody help me

Solution

The line that you want to change is a yAxis gridline. Since there is no API option to change a single gridline, you need to get access to the proper SVG element and change its attribute.

  chart: {
    events: {
      load: function() {
        const yAxis = this.yAxis[0];
        const firstTick = yAxis.ticks[yAxis.tickPositions[0]];

        firstTick.gridLine.attr({
          stroke: '#000',
          'stroke-width': 1
        });
      }
    }
  }

Live demo: https://jsfiddle.net/BlackLabel/hLasqxer/

API Reference: https://api.highcharts.com/class-reference/Highcharts.SVGElement#attr