Interactive iris regression

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body {
      font-family: sans-serif;
		}
    
    .line {
    	stroke: grey;
    	stroke-width: 3;
		}
    
    .popup {
      fill: red;
      font-weight: bold;
    }
  </style>
</head>

<body>
  <script>
    // Margin setup
    var margin = {
            top: 20,
            right: 20,
            bottom: 60,
            left: 60
        },
        width = 960 - margin.left - margin.right,
        height = 500 - margin.top - margin.bottom;

// Basic SVG canvas
    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 + ")");
    
    // Each species will have one of this array's colors
    var color = d3.scaleOrdinal(d3.schemeCategory10);
    
    // Format to display the coefficients of the regression equation
    var format = d3.format(".2f");
    
    // Function to move an element to the foreground
    d3.selection.prototype.moveToFront = function() {
      return this.each(function(){
        this.parentNode.appendChild(this);
      });
    };
    
    // Function to move an element to the background
    d3.selection.prototype.moveToBack = function() {
      return this.each(function() {
        var firstChild = this.parentNode.firstChild;
        if (firstChild) {
          this.parentNode.insertBefore(this, firstChild);
        } 
      });
    };
    
    // Function to give each species its symbol
    symbol = function(x){
      if (x=="setosa"){return d3.symbolCircle}
      else if (x=="versicolor"){return d3.symbolSquare}
      else if (x=="virginica"){return d3.symbolTriangle}
    };
    
    d3.csv("iris.csv", function(data) {
      data.forEach(function(d) {
        
        // Conversion into numbers
    		d.petal_length = +d.petal_length;
    		d.petal_width = +d.petal_width;
      });
      
      var x = d3.scaleLinear()
    		.domain([0,d3.max(data, function (d){return d.petal_length})])	
    		.range([0, width]);
      
      var y = d3.scaleLinear()
    		.domain([0,d3.max(data, function (d){return d.petal_width})])
    		.range([height, 0]);
      
      svg.append("g")
        .attr("transform", "translate(0," + height + ")")
        .call(d3.axisBottom(x))
      
      svg.append("g")
        .call(d3.axisLeft(y))
      
      // Label of the x-axis
      svg.append("text")
        .attr("transform",
            "translate(" + (width/2) + " ," + 
                           (height + margin.top + 20) + ")")
      	.attr("class","axis")
        .style("text-anchor", "middle")
        .text("Petal length");
      
      // Label of the y-axis
  		svg.append("text")
      	.attr("transform", "rotate(-90)")
        .attr("y", 0 - margin.left)
        .attr("x",0 - (height / 2))
        .attr("dy", "1em")
      	.attr("class","axis")
        .style("text-anchor", "middle")
        .text("Petal width");

var g = svg.selectAll("g").data(data).enter().append("g")
      	.attr("transform", function(d) {
        	return "translate(" + x(d.petal_length) + "," + y(d.petal_width) + ")"})
        .append("path")
        .attr("d", d3.symbol().type(function(d) {return symbol(d.species)}))
        .style("fill",function(d){return color(d.species)})
      
      	.on("mouseover", function(d){
        	d3.select(this).transition()
            .style("fill", "red")
          	.attr("d", d3.symbol()
                  .size(200)
                  .type(function(d) {return symbol(d.species)})
                 );

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