Observable Forum Question

Just looking into this issue.

I created that by simply placing the following code into a Custom HTML Block.

<style>
	svg {
		pointer-events: all;
	}
	.node circle {
	  fill: #FFF;
	  stroke: #009BE4;
	  stroke-width: 2px;
	}
	.node text {
	  font: 12px sans-serif;
	}
	.link {
	  fill: none;
	  stroke: #555;
	  stroke-width: 2px;
	  stroke-opacity: 0.3;
	}
</style>

<div id="box"></div>

<script src="https://d3js.org/d3.v5.min.js"></script>
<script>
  // Set the dimensions and margins of the diagram
  var margin = { top: 20, right: 90, bottom: 30, left: 90 },
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

  // declares a tree layout and assigns the size
  var treemap = d3.tree().size([height, width]);

  var flatData = [
    { name: "Mother Father", parent: null },
    { name: "Kid 1", parent: "Mother Father" },
    { name: "Kid 2", parent: "Mother Father" },
    { name: "Kid 11", parent: "Kid 1" },
    { name: "Kid 12", parent: "Kid 1" },
    { name: "Kid 21", parent: "Kid 2" },
    { name: "Kid 22", parent: "Kid 2" },
    { name: "Kid 211", parent: "Kid 21" },
    { name: "Kid 212", parent: "Kid 21" }
  ];

  // convert the flat data into a hierarchy
  var treeData = d3
    .stratify()
    .id(function(d) {
      return d.name;
    })
    .parentId(function(d) {
      return d.parent;
    })(flatData);

  // assign the name to each node
  treeData.each(function(d) {
    d.name = d.id;
  });

  // append the svg object to the body of the page
  // appends a 'group' element to 'svg'
  // moves the 'group' element to the top left margin
  var svg = d3
    .select("#box")
    .append("svg")
    .attr("width", width + margin.right + margin.left)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

  var i = 0,
    duration = 550,
    root;

  // Assigns parent, children, height, depth
  root = d3.hierarchy(treeData, function(d) {
    return d.children;
  });
  root.x0 = height / 2;
  root.y0 = 0;

  // Collapse after the second level
  root.children.forEach(collapse);

  update(root);

  // Collapse the node and all it's children
  function collapse(d) {
    if (d.children) {
      d._children = d.children;
      d._children.forEach(collapse);
      d.children = null;
    }
  }

  function update(source) {
    // Assigns the x and y position for the nodes
    var treeData = treemap(root);

    // Compute the new tree layout.
    var nodes = treeData.descendants().reverse(),
      links = treeData.descendants().slice(1);

    // Normalize for fixed-depth.
    nodes.forEach(function(d) {
      d.y = d.depth * 180;
    });

    // ****************** Nodes section ***************************

    // Update the nodes...
    var node = svg.selectAll('g.node').data(nodes, function(d) {
      return d.id || (d.id = ++i);
    });

    // Enter any new modes at the parent's previous position.
    var nodeEnter = node
      .enter()
      .append('g')
      .attr('class', 'node')
      .attr("transform", function(d) {
        return "translate(" + source.y0 + "," + source.x0 + ")";
      })
      .on('click', click);

    // Add Circle for the nodes
    nodeEnter
      .append('circle')
      .attr('class', 'node')
      .attr('r', 1e-6)
      .style("fill", function(d) {
        return d._children ? "lightsteelblue" : "#fff";
      });

    // Add labels for the nodes
    nodeEnter
      .append('text')
      .attr("dy", ".35em")
      .attr("x", function(d) {
        return d.children || d._children ? -13 : 13;
      })
      .attr("text-anchor", function(d) {
        return d.children || d._children ? "end" : "start";
      })
      .text(function(d) {
        return d.data.name;
      });

    // UPDATE
    var nodeUpdate = nodeEnter.merge(node);

    // Transition to the proper position for the node
    nodeUpdate
      .transition()
      .duration(duration)
      .attr("transform", function(d) {
        return "translate(" + d.y + "," + d.x + ")";
      });

    // Update the node attributes and style
    nodeUpdate
      .select('circle.node')
      .attr('r', 10)
      .style("fill", function(d) {
        return d._children ? "lightsteelblue" : "#fff";
      })
      .attr('cursor', 'pointer');

    // Remove any exiting nodes
    var nodeExit = node
      .exit()
      .transition()
      .duration(duration)
      .attr("transform", function(d) {
        return "translate(" + source.y + "," + source.x + ")";
      })
      .remove();

    // On exit reduce the node circles size to 0
    nodeExit.select('circle').attr('r', 1e-6);

    // On exit reduce the opacity of text labels
    nodeExit.select('text').style('fill-opacity', 1e-6);

    // ****************** links section ***************************

    // Update the links...
    var link = svg.selectAll('path.link').data(links, function(d) {
      return d.id;
    });

    // Enter any new links at the parent's previous position.
    var linkEnter = link
      .enter()
      .insert('path', "g")
      .attr("class", "link")
      .attr('d', function(d) {
        var o = { x: source.x0, y: source.y0 };
        return diagonal(o, o);
      });

    // UPDATE
    var linkUpdate = linkEnter.merge(link);

    // Transition back to the parent element position
    linkUpdate
      .transition()
      .duration(duration)
      .attr('d', function(d) {
        return diagonal(d, d.parent);
      });

    // Remove any exiting links
    var linkExit = link
      .exit()
      .transition()
      .duration(duration)
      .attr('d', function(d) {
        var o = { x: source.x, y: source.y };
        return diagonal(o, o);
      })
      .remove();

    // Store the old positions for transition.
    nodes.forEach(function(d) {
      d.x0 = d.x;
      d.y0 = d.y;
    });

    // Creates a curved (diagonal) path from parent to the child nodes
    function diagonal(s, d) {
      var path = `M ${s.y} ${s.x}
		C ${(s.y + d.y) / 2} ${s.x},
		  ${(s.y + d.y) / 2} ${d.x},
		  ${d.y} ${d.x}`;

      return path;
    }

    // Toggle children on click.
    function click(d) {
      if (d.children) {
        d._children = d.children;
        d.children = null;
      } else {
        d.children = d._children;
        d._children = null;
      }
      update(d);
    }
  }
</script>

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