D3 Examples - Center MB zoom circles in and out

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<!DOCTYPE html>
<meta charset="utf-8">
<style>
​
.background {
  fill: none;
  pointer-events: all; 
}
​
.feature {
  fill: #ccc;
  cursor: pointer;
}
​
.selected {
  fill: #c1c1c1;
}
  
</style>
<body>
<script src="https://d3js.org/d3.v4.min.js"></script>
  
<script>
// zoom in with image positioned at the center 
// irrespective of their location and size
// Aug 14, 2019  
  
var  active = d3.select(null);
var objh = 20;    
var width = window.innerWidth;
var height = window.innerHeight;
​
var cenx = width/2;
var ceny = height/2;
 
var jsonCircles = [
 { "x":  cenx-10, "y": ceny-40, "radius": 10, "color" : "limegreen" },
 { "x": cenx-30, "y": ceny, "radius": 20, "color" : "purple"},
 { "x": cenx+30, "y": ceny, "radius": 30, "color" : "orangered"},
 { "x": cenx+70, "y": ceny+30, "radius": 5, "color" : "skyblue"}
];
 
var zoom = d3.zoom()
    .on("zoom", zoomed);
  
var initialTransform = d3.zoomIdentity
    .translate(0,0)
    .scale(1.0);
​
var svg = d3.select("body").append("svg")
     
 svg.attr("width", width)
    .attr("height", height)
    .on("click", stopped, true);
 
 svg.append("rect")
    .attr("class", "background")
    .attr("width", width)
    .attr("height", height)
        .style("fill", "white")
        .style("stroke", "black")
    .on("click", reset);
  
var g = svg.append("g");
​
svg.call(zoom) // delete this line to disable free zooming
   .call(zoom.transform, initialTransform);
​
var circles = g.selectAll("circle")
               .data(jsonCircles)
               .enter()
               .append("circle")
               .attr("cx", function (d) { return d.x; })
               .attr("cy", function (d) { return d.y; })
               .attr("r", function (d) { return d.radius;})
               .style("fill", function(d) { return d.color;})
                         .attr("class", "feature")
                         .on("click", clicked);;
  
function clicked(d) {
//  if (active.node() === this) return reset();
  if (active.node() === this) {
   d3.select(this).style("fill", d.color);
   return reset();
  } 
  
  active.classed("active", false);
  active = d3.select(this).classed("active", true);
  
  dr = d.radius
  dx = d.radius
  dy = d.radius
  x = d.x 
  y = d.y
​
//  active.attr("class", "selected")  
//  scale = Math.max(1, Math.min(height/objh/2.05, 0.9 / Math.max(dx / width, dy / height)))
  
  scale = height/dr/2.05  // Same zoom level to all objects 
  translate = [width / 2 - scale * x, height / 2 - scale * y];
    
//  console.log("width");
  
  var transform = d3.zoomIdentity
    .translate(translate[0], translate[1])
    .scale(scale);
​
  svg.transition()
     .duration(1000)
     .call(zoom.transform, transform);
​
  d3.select(this).style("fill", "black");
}
​
function reset() {
//  d3.select(this).style("fill", "blue");
  active.classed("active", false);
  active = d3.select(null);
​
  svg.transition()
      .duration(750)
      .call(zoom.transform, initialTransform);
}
​
function zoomed() {
  var transform = d3.event.transform; 
​
  g.style("stroke-width", 1.5 / transform.k + "px");
  g.attr("transform", transform);
}
​
// If the drag behavior prevents the default click,
// also stop propagation so we don’t click-to-zoom.
function stopped() {
  if (d3.event.defaultPrevented) d3.event.stopPropagation();
}
​
</script>
​
    