S curve

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

<body>
  <script>
    
    var points = [[150, 50], [150, 200]]
    
    var svg = d3.select("body").append("svg")
      .attr("width", 300)
      .attr("height", 300)

svg.append("path")
      .datum(points)
      .attr("stroke", "grey")
    	.attr("stroke-width", "15px")
    	.attr("fill", "none")
    	.attr("d", function(d){
      	
      	let x1 = d[0][0]
        let x2 = d[1][0]
        let y1 = d[0][1]
        let y3 = d[1][1]
        let yGap = (y3 - y1)
        let xPadding = yGap/3
        let y2 = y1 + (yGap/2)
        
				let M = "M" + x1 + " " + y1 + " "
        let C1 = "C" + (x1 + xPadding) + " " + y1 + ", "
        let C2 = (x1 + xPadding) + " " + y2 + ", "
        let C3 = x1 + " " + y2 + " "
        let S = "S " + (x2 - xPadding) + " " + y3 + ", "
        let end = x2 + " " + y3
        
        return M + C1 + C2 + C3 + S + end
        
      })
    
      svg.selectAll("circle")
        .data(points).enter()
        .append("circle")
        .attr("cx", d => d[0])
        .attr("cy", d => d[1])
        .attr("r", 10)
        .attr("fill", "black")

</script>
</body>

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