Appending circle by the positions of parent and child node d3js - d3.js

Question

This is my treeMap right now:
I want to append a button between the nodes. The nodes have parents and child relationships. The 2 white circle with plus sign are children of the coursera picture. So I am trying to append the button by calling a function that takes the parent's (x,y) and the children(x,y), then create a circle there probably.
Here is my full code, I did not write most of this code so I am not understanding things fully. I recognize that the link path between nodes were drawn by this function: ` 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;
}`
How would like append a circle between 2 nodes in my code?
Full code: `
import React, { Component } from "react";
import * as d3 from "d3";
import { hierarchy, tree } from "d3-hierarchy";
import "../Tree.css";
//import "./Tree.css";
class Tree extends Component {
constructor(props) {
super(props);
this.state = { collapse: false, text: "hi", visible: true };
//this.toggle = this.toggle.bind(this);
}
handleChange = d => {
this.props.on_click_change(d);
};
componentDidMount() {
var that = this;
var treeData = this.props.roadmapData;
// Set the dimensions and margins of the diagramS
var height1 = window.innerHeight;
var margin = { top: 0, right: 0, bottom: 0, left: 0 },
width = 1080 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
// 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("li")
.append("svg")
.call(
d3.zoom().on("zoom", function() {
svg.attr("transform", d3.event.transform);
})
)
.attr("width", 1800 - margin.right - margin.left)
.attr("height", 900 - margin.top - margin.bottom)
.append("g")
.attr("transform", "translate(" + +"," + margin.top + ")");
var i = 0,
duration = 500,
root;
// declares a tree layout and assigns the size
var treemap = d3.tree().size([window.innerHeight, window.innerWidth]);
root = d3.hierarchy(treeData, function(d) {
return d.children;
});
root.x0 = height / 2;
root.y0 = 0;
console.log(this.props.treeData);
// 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(),
links = treeData.descendants().slice(1),
more_button = treeData.descendants();
// 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")
//if deleted, bubbles come from the very top, is weird
.attr("transform", function(d) {
return "translate(" + source.y0 + "," + source.x0 + ")";
});
// 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", 0)
.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;
});
*/
var diameter = 30;
nodeEnter
.append("image")
.on("click", click)
.attr("xlink:href", function(d) {
return d.data.website_image;
})
.attr("height", diameter * 2)
.attr("transform", "translate(-30," + -30 + ")");
// 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", diameter)
.style("fill", function(d) {
return d._children ? "lightsteelblue" : "#fff";
})
.attr("cursor", "pointer");
nodeUpdate
.append("circle")
.on("click", click2)
.attr("additional", "extra_circle")
.attr("r", 10)
.attr("transform", "translate(0," + -40 + ")");
// 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")
.style("fill", "red")
.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) {
console.log(d, d.parent);
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);
}
function click2(d) {
console.log(d.data.name);
that.setState({ text: d.data.details });
that.handleChange(d);
}
}
}
render() {
return null;
}
}
export default Tree;
`

Answer 1

Thanks to Coola, here's the solution:
`nodeEnter
.append("circle")
.attr("class", "extra_info")
.on("click", function(d) {})
.attr("cy", function(d) {
if (d.parent != null) {
d.x_pos = d.x;
d.parent_x_pos = d.parent.x;
}
if (d.parent_x_pos != null) {
return (d.x_pos + d.parent_x_pos) / 2 - d.x_pos;
}
})
.attr("cx", -90)
.attr("r", 7);`
So basically whenever you do function(d){} instead of just calling a single thing, the d variable gets passed and it contains a lot of information, including the parent of the current element and the child if there is one, and the x y positions, etc. d refers to the current element we are at. So we can comfortably use d.parent.x and d.x to calculate positions, here's what it looks like now:
Btw, d3 and svg's x and y seems to be reversed sometimes. As you can see: I am deciding cy, SVG circle's attribute on determining its y position by d.x.