Scatterplot Tutorial

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

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

.dot {
  stroke: #000;
}

.tooltip {
  position: absolute;
  width: 200px;
  height: 28px;
  pointer-events: none;
}
</style>
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>

/* 
 * value accessor - returns the value to encode for a given data object.
 * scale - maps value to a visual display encoding, such as a pixel position.
 * map function - maps from data value to display value
 * axis - sets up axis
 */

// setup x 
var xValue = function(d) { return d.Calories;}, // data -> value
    xScale = d3.scale.linear().range([0, width]), // value -> display
    xMap = function(d) { return xScale(xValue(d));}, // data -> display
    xAxis = d3.svg.axis().scale(xScale).orient("bottom");

// setup y
var yValue = function(d) { return d["Protein (g)"];}, // data -> value
    yScale = d3.scale.linear().range([height, 0]), // value -> display
    yMap = function(d) { return yScale(yValue(d));}, // data -> display
    yAxis = d3.svg.axis().scale(yScale).orient("left");

// setup fill color
var cValue = function(d) { return d.Manufacturer;},
    color = d3.scale.category10();

// add the graph canvas to the body of the webpage
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 + ")");

// add the tooltip area to the webpage
var tooltip = d3.select("body").append("div")
    .attr("class", "tooltip")
    .style("opacity", 0);

// load data
d3.csv("cereal.csv", function(error, data) {

// change string (from CSV) into number format
  data.forEach(function(d) {
    d.Calories = +d.Calories;
    d["Protein (g)"] = +d["Protein (g)"];
//    console.log(d);
  });

// don't want dots overlapping axis, so add in buffer to data domain
  xScale.domain([d3.min(data, xValue)-1, d3.max(data, xValue)+1]);
  yScale.domain([d3.min(data, yValue)-1, d3.max(data, yValue)+1]);

// x-axis
  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("Calories");

// y-axis
  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("Protein (g)");

// draw dots
  svg.selectAll(".dot")
      .data(data)
    .enter().append("circle")
      .attr("class", "dot")
      .attr("r", 3.5)
      .attr("cx", xMap)
      .attr("cy", yMap)
      .style("fill", function(d) { return color(cValue(d));}) 
      .on("mouseover", function(d) {
          tooltip.transition()
               .duration(200)
               .style("opacity", .9);
          tooltip.html(d["Cereal Name"] + "<br/> (" + xValue(d) 
	        + ", " + yValue(d) + ")")
               .style("left", (d3.event.pageX + 5) + "px")
               .style("top", (d3.event.pageY - 28) + "px");
      })
      .on("mouseout", function(d) {
          tooltip.transition()
               .duration(500)
               .style("opacity", 0);
      });

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

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

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

</script>
</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

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