addElements

<!DOCTYPE html>

<html>

<style type="text/css">

body {
        margin: 0;
        font-family: sans-serif;
        font-size: 11px;
      }

.axis path, .axis line {
        fill: none;
        stroke: black;
        shape-rendering: crispEdges;  /* Round any decimal pixels so it'll render nicely */
      }

/*
      //Can use CSS3 Transitions, but not for everything (e.g. change radius size)
      circle:hover{
        fill: green;
      }
      */

</style>
  </head>

<body>
    
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script type="text/javascript">
      var w = window.innerWidth,
          h = window.innerHeight,
          margin = { top: 40, right: 20, bottom: 20, left: 40 },
          radius = 6;

var svg = d3.select("body").append("svg").attr({
        width: w,
        height: h
      });

var dataset = [
        { x: 100, y: 110 },
        { x: 83, y: 43 },
        { x: 92, y: 28 },
        { x: 49, y: 74 },
        { x: 51, y: 10 },
        { x: 25, y: 98 },
        { x: 77, y: 30 },
        { x: 20, y: 83 },
        { x: 11, y: 63 },
        { x:  4, y: 55 },
        { x:  0, y:  0 },
        { x: 85, y: 100 },
        { x: 60, y: 40 },
        { x: 70, y: 80 },
        { x: 10, y: 20 },
        { x: 40, y: 50 },
        { x: 25, y: 31 }
      ];

// We're passing in a function in d3.max to tell it what we're maxing (x value)
      var xScale = d3.scaleLinear()
          .domain([0, d3.max(dataset, function (d) { return d.x + 10; })])
          .range([margin.left, w - margin.right]);  // Set margins for x specific

// We're passing in a function in d3.max to tell it what we're maxing (y value)
      var yScale = d3.scaleLinear()
          .domain([0, d3.max(dataset, function (d) { return d.y + 10; })])
          .range([margin.top, h - margin.bottom]);  // Set margins for y specific

// Add a X and Y Axis (Note: orient means the direction that ticks go, not position)
      var xAxis = d3.axisBottom(xScale).tickFormat(function (d) { return d.x; });
      var yAxis = d3.axisLeft(yScale);

// Sets circles attributes
      var circleAttrs = {
          cx: function(d) { return xScale(d.x); },
          cy: function(d) { return yScale(d.y); },
          r: radius
      };

// Adds X-Axis as a 'g' element
      var xAxisGroup = svg.append("g").attr({
        "class": "axis",  // Give class so we can style it
        transform: "translate(" + [0, margin.top] + ")"  // Translate just moves it down into position (or will be on top)
      }).call(xAxis);  // Call the xAxis function on the group

// Adds Y-Axis as a 'g' element
      var yAxisGroup = svg.append("g").attr({
        "class": "axis",
        transform: "translate(" + [margin.left, 0] + ")"
      }).call(yAxis);  // Call the yAxis function on the group

var drag = d3.drag()
      	.on('dragstart', function(d, i) {
          d3.event.sourceEvent.stopPropagation();
          d3.select("#t" + d.x + "-" + d.y + "-" + i).remove();
        })
      	.on('drag', dragged)
      	.on('dragend', dragended);
      
      var circles = svg.selectAll("circle")
          .data(dataset)
          .enter()
          .append("circle")
          .attr(circleAttrs)  // Get attributes from circleInitialAttrs var
          .on("mouseover", handleMouseOver)
          .on("mouseout", handleMouseOut)
      		.call(drag);

// On Click, we want to add data to the array and chart
      svg.on("click", function() {
        if(d3.event.defaultPrevented) return;
          var coords = d3.mouse(this);

// Normally we go from data to pixels, but here we're doing pixels to data
          var newData= {
            x: Math.round( xScale.invert(coords[0])),  // Takes the pixel number to convert to number
            y: Math.round( yScale.invert(coords[1]))
          };

dataset.push(newData);   // Push data to our array

xScale.domain([0, d3.max(dataset, function (d) { return d.x + 10; })])
          yScale.domain([0, d3.max(dataset, function (d) { return d.y + 10; })])

// Update Axis (e.g. might increase if new higher value added)
          xAxisGroup.transition().call(xAxis);  // Update X-Axis
          yAxisGroup.transition().call(yAxis);  // Update Y-Axis

// When adding new items, goes from 0,0 and transition to place
          var c = svg.selectAll("circle")  // For new circle, go through the update process
            .data(dataset);

// Updates existing circles to new positions by just adding attr
          c.transition()
            .ease("elastic")
            .attr(circleAttrs)

c.enter()
            .append("circle")
            .attr(circleAttrs)
            .on("mouseover", handleMouseOver)
            .on("mouseout", handleMouseOut)
          	.call(drag);
      })

// Create Event Handlers for mouse
      function handleMouseOver(d, i) {  // Add interactivity

// Use D3 to select element, change color and size
            d3.select(this).attr({
              fill: "orange",
              r: radius * 2
            });

// Specify where to put label of text
            svg.append("text").attr({
               id: "t" + d.x + "-" + d.y + "-" + i,  // Create an id for text so we can select it later for removing on mouseout
                x: function() { return xScale(d.x) - 30; },
                y: function() { return yScale(d.y) - 15; }
            })
            .text(function() {
              return [d.x, d.y];  // Value of the text
            });
          }

function handleMouseOut(d, i) {
            // Use D3 to select element, change color back to normal
            d3.select(this).attr({
              fill: "black",
              r: radius
            });

// Select text by id and then remove
            d3.select("#t" + d.x + "-" + d.y + "-" + i).remove();  // Remove text location
          }
      
        function dragged(d,i) {
        if(d3.event.x >= margin.left && d3.event.y >= margin.top
          	&& d3.event.x <= w 
          	&& d3.event.y <= h) {
        d.x = d3.event.x;
        d.y = d3.event.y;
        console.log(d3.event.x + " " + d3.event.y);
         d3.select(this)
            .attr("cx", d.x )
            .attr("cy", d.y );
        }
			}
      
      function dragended(d, i) {
        console.log("end");
        var newX = Math.round(xScale.invert(d.x)),
            newY = Math.round(yScale.invert(d.y));
        dataset[i].x = newX;
        dataset[i].y = newY;
      }

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

Modified http://d3js.org/d3.v4.min.js to a secure url

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