Because it’s not simple or intuitive.

D3.js doesn’t have a function like drawLine(x, y) for you to effortlessly draw a line.

Why is D3.js like that? I don’t know.

So here I am, this week, writing a short and simple tutorial/guide on how to draw a line in D3.js. Version 5 (instead of 4) will be used.

Setup

Inline Javascript and CSS will be used in a single .html file. And D3.js will be fetched from a content delivery network. So, the .html file that you would use will have this structure:

<script src="https://d3js.org/d3.v5.js"></script>
<svg id="svg1" style="margin: 0 auto; display: block;"></svg>
<script>
</script>

svg element configuration

Inside the Javascript portion, use the d3.select function to obtain a reference to the SVG element. Its argument is a string that contains the element’s id attribute.

const svg = d3.select('#svg1');

The value returned by d3.select('#svg1') has functions that are appended in a chaining fashion to further modify the selected SVG element. This chain-appendment fashion is used throughout many D3.js’s operations and is the key to using it.

For instance, it seems that you can modify the HTML attributes and CSS properties of a SVG element in Javascript with the attr and style functions.

Below, an element width and height of 400 pixels is defined, with a black background color.

const svg = d3.select('#svg1')
    .attr('width', 400)
    .attr('height', 400)
    .style('background-color', 'black');

Defining a line

Now we will add a line to the SVG element. We will use the append function of the svg variable we defined.

Line color

We want to create a light green colored line. We define a color using the style function to modify a stroke property, which is the color of the line here.

svg.append('line')
    .style("stroke", "lightgreen");

Line width

We also want to define the width of the line with the stroke-width property. A line of ten pixels wide is defined below.

svg.append('line')
    .style("stroke", "lightgreen")
    .style("stroke-width", 10);

Line location

For a simple two-dimensional line, the endpoints of the line must be defined for it to be displayed. This is achieved with the x1, y1, x2, and y2 attributes. x1 and y1 correspond to the first endpoint while x2 and y2 correspond to the second one.

D3.js seems to use a coordinate system with the origin at the top-left corner.

svg.append('line')
    .style("stroke", "lightgreen")
    .style("stroke-width", 10)
    .attr("x1", 0)
    .attr("y1", 0)
    .attr("x2", 200)
    .attr("y2", 200); 

The result

That will be all to create the line that we want.

Full source code

The full source code for your convenience:

<script src="https://d3js.org/d3.v5.js"></script>
<svg id="svg1" style="margin: 0 auto; display: block;"></svg>
<script>
    let svg = d3.select('#svg1')
        .attr('width', 400)
        .attr('height', 400)
        .style('background-color', 'black');

    svg.append('line')
        .style("stroke", "lightgreen")
        .style("stroke-width", 10)
        .attr("x1", 0)
        .attr("y1", 0)
        .attr("x2", 200)
        .attr("y2", 200);        
</script>

Remarks

Nothing follows.