Draggable Connected Dot Plot

Learning how to use d3-drag for an interactive "Make a guess" connected dot plot. Dragging is limited to the x-axis and the ends of the line cannot cross over (i.e. their positions cannot be switched). Clicking reset returns the dots to their original positions.

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    .dot {
      fill: white;
      stroke: black;
    }
    
    .active {
      stroke: red;
    }
    
    body {
      font-family: monospace;
    }
    
    a {
      position: absolute;
      top: 20px;
      left: 20px;
    }
  </style>
</head>

<body>
  <a href="#" class="reset-btn">Reset</a>
  <script>
    var margin = {top: 0, right: 0, bottom: 0, left: 0};
    
    var width = 960 - margin.left - margin.right,
    		height = 500 - margin.top - margin.bottom;
    
    var svg = d3.select("body").append("svg")
          .attr("width", width + margin.left + margin.right)
          .attr("height", height + margin.top + margin.bottom)
        .append("g")
          .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
    
    var radius = 20,
        strokewidth = 5;
    
    var te = d3.easeCubic;
    
    var dots = [
      {
        type: "min",
        colour: "white",
        x: width / 4,
        y: height / 2
      },
      { 
        type: "median",
        colour: "grey",
				x: width / 2,
        y: height / 2
      },
      {
        type: "max",
        colour: "black",
        x: width * 3 / 4,
        y: height / 2
    	}];
    
    var dotsoriginal = JSON.parse(JSON.stringify(dots));
    
    var dragbehaviour = d3.drag()
    	.on("start", dragstarted)
    	.on("drag", dragged)
    	.on("end", dragended);			
    
    var labels = svg.selectAll("text")
    	.data(dots)
    .enter().append("text")
    	.attr("x", function(d) { return d.x })
    	.attr("y", function(d) { 
        	return height / 2 + (d.type == "min" || d.type == "max" ? 60 : -50)
      })
    	.attr("text-anchor", "middle")
    	.text(function(d) { return "(" + d.x + ", " + d.y + ")" });
      
    var lineGenerator = d3.line()
    	.x(function(d) { return d.x + (d.type == "min" ? radius : -radius)})
    	.y(function(d) { return d.y });
    
    var pathString = function() { return lineGenerator(dots) };
    
    var line = svg.append("path")
    	.attr("class", "line")
    	.attr("d", pathString)
    	.attr("stroke", "black")
    	.style("stroke-width", strokewidth);
    
    var circles = svg.selectAll("circle")
    	.data(dots)
		.enter().append("circle")
    	.attr("class", "dot")
    	.attr("cx", function(d) { return d.x; })
    	.attr("cy", function(d) { return d.y; })
    	.attr("r", radius)
    	.style("fill", function(d) { return d.colour })
    	.style("stroke-width", strokewidth)
    	.call(dragbehaviour);
    
    function dragstarted(d) {
      d3.select(this)
        .classed("active", true);
    }
    
    function dragged(d) {
      var minX, maxX;
      
      if (d.type == "min") {
        minX = radius + strokewidth;
        maxX = dots[1].x - (radius * 2) - strokewidth;
      } else if (d.type == "median") {
        minX = dots[0].x + (radius * 2) + strokewidth;
        maxX = dots[2].x - (radius * 2) - strokewidth;
      } else {
        minX = dots[1].x + (radius * 2) + strokewidth;
        maxX = width - radius - strokewidth;
      }
      
      d3.select(this)
      	.attr("cx", d.x = Math.max(minX, Math.min(maxX, d3.event.x)));
      
      labels
    		.attr("x", function(d) { return d.x })
        .text(function(d) { return "(" + d.x + ", " + d.y + ")" });
      
      line.attr("d", pathString);      
    }
    
    function dragended(d) {
      d3.select(this)
      	.classed("active", false);
    }
    
    d3.select(".reset-btn")
    	.on("click", function(d) {
      	d3.event.preventDefault();
      	dots = dotsoriginal;
      	dotsoriginal = JSON.parse(JSON.stringify(dots));
      	
     	 	line.transition()
        	.duration(500)
        	.ease(te)
          .attr("d", pathString); 
      
      	circles.data(dots).transition()
        	.duration(500)
        	.ease(te)
        	.attr("cx", function(d) { return d.x; }); 
      
      	labels.data(dots).transition()
        	.duration(500)
        	.ease(te)
        	.attr("x", function(d) { return d.x; })
      		.text(function(d) { return "(" + d.x + ", " + d.y + ")" });
    	});

</script>
</body>

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