[Solved]-How to make legend color show up in Highcharts for a line graph under certain conditions?-Highcharts

In your HTML file, add:

<script src="https://code.highcharts.com/maps/modules/map.src.js"></script>

Above your chart code add:

Highcharts.addEvent(Highcharts.Chart.prototype, 'render', function() {
    this.series.forEach(function(series) {
      if (series.legendLine) {
        var pathDef = series.legendLine.attr('d').split(' ');
        pathDef[5] = parseFloat(pathDef[2]) + 0.0001;
        series.legendLine.attr({
          d: pathDef
        });
      }
    })
  });

The fix is in adding a smidge to your path definition (you can test this by going into the svg path element in dev tools and changing the last number in the 'd' attribute to a float:

<path fill="none" d="M 0 11 L 16 11" ... ></path>

becomes:

<path fill="none" d="M 0 11 L 16 11.1" ... ></path>

The function above goes through each series and massages the line data adding the fractional value in the right place. Total hack. However why the line does not appear if it does have this fractional value, I don't know. I would also test this across browsers to see that it works consistently as it may not (I tested in Chrome).

Here is a somewhat related issue where I cribbed the fix: https://www.highcharts.com/forum/viewtopic.php?t=38588 (Most of it is addressing a completely different problem, but the legend lines did show up so I investigated why).

If you don't want to use the extra library, you could probably write a native js function to find the legend lines and manipulate them in the same way.

In addition to @Katharine's solution, here's another I discovered.

If I enable the marker in one series (setting the radius to 0) and then put the data in a second series which is linked to the previous, it works. (If I do it all in one series, there is again no legend color.)

series: [{
    name: 'purple', lineWidth: 3, color: {
        linearGradient: [ 00, 00, 00, 350 ],
        //linearGradient: { x1: 0, y1: 0, x2: 0, y2: 1, },
        stops: [
            [0, 'purple'], // start 
            [1, 'black'] // end
        ], 
    }, 
    data: [ 100, 95, 80, 60, 35, 50, 20, 10, 3, 2, 30, 40, ],
},{
    marker: {
        enabled: true,
        radius: 0,
    },
    name: 'green', color: 'green', lineWidth: 3, 
},{
    linkedTo: ':previous',
    lineWidth: 3, color: {
        //linearGradient: [ 00, 00, 00, 350 ], 
        linearGradient: { x1: 0, y1: 0, x2: 0, y2: 1, },
        stops: [
            [0, 'green'], // start 
            [1, 'black'] // end
        ], 
    }, 
    data: [ 100, 100, 95, 80, 60, 35, 50, 20, 10, 3, 2, 30, ],
}],

Here's the "fixed" fiddle: https://jsfiddle.net/jwinkle/wztq78n1/6/

I reported the original issue as a possible bug to Highcharts, and they confirmed that it's an SVG rendering problem out of their control.

Here, you can find a nice explanation of the problem.

The difference results from the fact that Highcharts uses 'userSpaceOnUse' as a default value for gradientUnits if a gradient is defined as an array.

Highcharts source code:

// Keep < 2.2 kompatibility
if (isArray(gradAttr)) {
  (colorOptions as any)[gradName] = gradAttr = {
    x1: gradAttr[0] as number,
    y1: gradAttr[1] as number,
    x2: gradAttr[2] as number,
    y2: gradAttr[3] as number,
    gradientUnits: 'userSpaceOnUse'
  };
}

As a possible solution, you can set gradientUnits also for linearGradient defined as an object:

color: {
  linearGradient: {
    ...,
    gradientUnits: 'userSpaceOnUse'
  },
  ...
}

Live demo: http://jsfiddle.net/BlackLabel/yz6sagxj/

Or modify the paths very slightly so that they are not perfectly horizontal

chart: {
  events: {
    render: function() {
      this.series.forEach(function(series) {
        if (series.legendLine) {
          var pathDef = series.legendLine.attr('d').split(' ');
          pathDef[5] = parseFloat(pathDef[2]) + 0.0001;
          series.legendLine.attr({
            d: pathDef
          });
        }
      })
    }
  }
}

Live demo: http://jsfiddle.net/BlackLabel/L1kzg0cn/

Github thread: https://github.com/highcharts/highcharts/issues/1936