d3 Opacity

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
    const data_points = [
      { x: 100, y: 300, type: 'apple', mass: 80, percentEaten: 70 },
      { x: 200, y: 100, type: 'banana',  mass: 105, percentEaten: 50 },
      { x: 300, y: 200, type: 'grape',  mass: 7, percentEaten: 30 },
      { x: 400, y: 200, type: 'grape',  mass: 5, percentEaten: 20 },
      { x: 500, y: 400, type: 'banana',  mass: 120, percentEaten: 85 },
    ]
       
		const colorScale = d3.scaleOrdinal()
    	.domain(['apple', 'banana', 'grape'])
    	.range(['red', 'yellow', 'purple'])
   
    const massScale = d3.scaleLinear()
    	.domain([5, 120])
    	.range([5, 15])
    
    const eatenScale = d3.scaleLinear()
    	.domain([0, 100])
    	.range([1, 0])
    
    const svg = d3.select("body").append("svg")
      .attr("width", 600)
      .attr("height", 600)
    	.style("background-color", "skyblue")

svg.selectAll('circle')
    	.data(data_points)
    	.enter()
    	.append('circle')
    	.attr('r', d => massScale(d.mass))
    	.attr('cx', d => d.x)
      .attr('cy', d => d.y)
    	.attr('fill', d => colorScale(d.type))
    	.attr('fill-opacity', d => eatenScale(d.percentEaten))
  </script>
</body>

https://d3js.org/d3.v4.min.js