Where is d3.svg.diagonal()?

D3 version 4.9.0 introduced link shapes, which have the same functionality of the old d3.svg.diagonal in D3 v3.

According to the API:

The link shape generates a smooth cubic Bézier curve from a source point to a target point. The tangents of the curve at the start and end are either vertical, horizontal or radial.

There are three methods:

• d3.linkHorizontal()
• d3.linkVertical()
• d3.linkRadial()

So, for a collapsible tree like that one you linked, you define the path d attribute as:

.attr("d", d3.linkHorizontal()
    .x(function(d) { return d.y; })
    .y(function(d) { return d.x; }));

Demo:

Suppose you have an object with source and target, each one with x and y properties:

var data = {
  source: {
    x: 20,
    y: 10
  },
  target: {
    x: 280,
    y: 100
  }
};

First, you create the link generator:

var link = d3.linkHorizontal()
  .x(function(d) {
    return d.x;
  })
  .y(function(d) {
    return d.y;
  });

And then you can draw the path just by passing that data to the link generator:

.attr("d", link(data))

Here is the demo:

var svg = d3.select("svg");

var data = {
  source: {
    x: 20,
    y: 10
  },
  target: {
    x: 280,
    y: 100
  }
};

var link = d3.linkHorizontal()
  .x(function(d) {
    return d.x;
  })
  .y(function(d) {
    return d.y;
  });

svg.append("path")
  .attr("d", link(data))
  .style("fill", "none")
  .style("stroke", "darkslateblue")
  .style("stroke-width", "4px");

<script src="https://d3js.org/d3.v5.min.js"></script>
<svg></svg>