Simple linear regression

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.dot {
  stroke: #000;
}

</style>
<body>
    
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

function speciesSelect(array,species){
 return array.filter(function(d){
   return d.species===species;
 });
}

function residuals(array,y,x,regOutput){
  array.forEach(function(d) {
   d.residual = d[y]-d[x]*regOutput.beta-regOutput.alpha;
  });
}

function regressionOutput(array,x,y){
  var meanX = mean(array,x);
  var meanY = mean(array,y);
  var dotProductXY = dotProduct(array,x,y);
  var dotProductXX = dotProduct(array,x,x);
  var beta = (dotProductXY-meanX*meanY)/(dotProductXX-meanX^2);
  var alpha = meanY - beta*meanX;
  return {'alpha':alpha,'beta':beta};
}

function dotProduct(array,x,y){
  var result = 0;
  array.forEach(function(d) {
    result += d[x]*d[y];
  });
  return result;
}

function mean(array,x){
  var result = 0;
  array.forEach(function(d) {
    result += d[x];
  });
  result=result/array.length;
  return result;
}

var margin = {top: 20, right: 20, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scale.linear()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var color = d3.scale.category10();

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

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 + ")");

d3.tsv("data.tsv", function(error, data) {
  if (error) throw error;

data.forEach(function(d) {
    d.sepalLength = +d.sepalLength;
    d.sepalWidth = +d.sepalWidth;
  });

var species=["setosa","virginica","versicolor"];
  var xlimits=d3.extent(data, function(d) { return d.sepalWidth;}) 
  xlimits[0]=0;
  var ylimits=d3.extent(data, function(d) { return d.sepalLength;})
  ylimits[0]=0;

x.domain(xlimits).nice();
  y.domain(ylimits).nice();

var regressionstats=[];
  species.forEach(function(d){
    regressionstats.push(regressionOutput(speciesSelect(data,d),"sepalWidth","sepalLength"));
    });

var lineDataMotherload=[];
  regressionstats.forEach(function(d){
    lineDataMotherload.push([ { "x": xlimits[0],   "y": d.alpha},  { "x": xlimits[1],
      "y":d.alpha+d.beta*xlimits[1]}]);
  });

var lineFunction = d3.svg.line()
                      .x(function(d) { return x(d.x); })
                      .y(function(d) { return y(d.y); })
                      .interpolate("linear");

var linePlots = lineDataMotherload.map(lineFunction);

var lineGraph = svg.selectAll(".line")
                            .data(linePlots)
                            .enter().append("path")
                            .attr("d",function(d){return d;})
                            .attr("stroke",function(d,i){return color(species[i]);})
                            .attr("stroke-width", 2)
                            .attr("fill", "none")
                            .attr("opacity",.5);

svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis)
    .append("text")
      .attr("class", "label")
      .attr("x", width)
      .attr("y", -6)
      .style("text-anchor", "end")
      .text("Sepal Width (cm)");

svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("class", "label")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Sepal Length (cm)")

svg.selectAll(".dot")
      .data(data)
.attr("d",function(d){console.log(d);})
    .enter().append("circle")
      .attr("class", "dot")
      .attr("r", 3.5)
      .attr("cx", function(d) { return x(d.sepalWidth); })
      .attr("cy", function(d) { return y(d.sepalLength); })
      .style("fill", function(d) { return color(d.species); });

var legend = svg.selectAll(".legend")
      .data(color.domain())
    .enter().append("g")
      .attr("class", "legend")
      .attr("transform", function(d, i) { return "translate(0," + i * 20 + ")"; });

legend.append("rect")
      .attr("x", width - 18)
      .attr("width", 18)
      .attr("height", 18)
      .style("fill", color);

legend.append("text")
      .attr("x", width - 24)
      .attr("y", 9)
      .attr("dy", ".35em")
      .style("text-anchor", "end")
      .text(function(d) { return d; });

});

</script>

https://d3js.org/d3.v3.min.js