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How to create a line chart using D3

Shahpar Khan

A bit about D3

D3 is an interactive JavaScript library for data visualization. It uses Scalar Vector Graphics (SVG) coupled with HTML and CSS to display charts and figures that illustrate the numeric data. You can also use D3 to make line charts. Here is a step-by-step guide on how to make a line chart using D3.

Step 1: Dataset

Before even starting to code, we need a data set to base our chart on. For this example, we will take a basic 2D array of random numbers. Our array is:

var dataset1 = [
            [1,1], [12,20], [24,36],
            [32, 50], [40, 70], [50, 100],
            [55, 106], [65, 123], [73, 130],
            [78, 134], [83, 136], [89, 138],
            [100, 140]
        ];

Step 2: D3 and SVG container

The first thing we need to do is import the D3 script using the src attribute and then initialize our SVG container with the appropriate width and height:

<script src="https://d3js.org/d3.v4.min.js"></script>
<svg width="500" height="400"></svg>

Step 3: Set margin

To make the chart look more centered and clear, we need to set a margin for our SVG. We are making four variables; svg, margin, width, and height. svg is initialized with the 500px width and 400px height. These widths and heights are then adjusted according to the 200px margin.

var svg = d3.select("svg"),
            margin = 200,
            width = svg.attr("width") - margin,
            height = svg.attr("height") - margin

Step 4: Set scale

For discrete data visualization on the x-axis, we construct a Linear Scale or scaleLinear(). scaleLinear() uses the linear equation (y = mx + c) to interpolate domain and range across the axis:

var xScale = d3.scaleLinear().domain([0, 100]).range([0, width]),
            yScale = d3.scaleLinear().domain([0, 200]).range([height, 0]);

Step 5: Add text

We need to add a title, x-axis label, and y-axis label to our plot. For this purpose, we first append text to our svg, then we set the position, style, and actual text attribute. To rotate our text for the y-axis, we use transform and specify the angle of rotation.

// Step 5
        // Title
        svg.append('text')
        .attr('x', width/2 + 100)
        .attr('y', 100)
        .attr('text-anchor', 'middle')
        .style('font-family', 'Helvetica')
        .style('font-size', 20)
        .text('Line Chart');
        
        // X label
        svg.append('text')
        .attr('x', width/2 + 100)
        .attr('y', height - 15 + 150)
        .attr('text-anchor', 'middle')
        .style('font-family', 'Helvetica')
        .style('font-size', 12)
        .text('Independant');
        
        // Y label
        svg.append('text')
        .attr('text-anchor', 'middle')
        .attr('transform', 'translate(60,' + height + ')rotate(-90)')
        .style('font-family', 'Helvetica')
        .style('font-size', 12)
        .text('Dependant');

Step 6: Add axis

Now, we need to add both of the axes. For the x-axis, we call d3.axisBottom because we need to align it at the bottom of the canvas. For the y-axis, we call d3.axisLeft because we want to align it to the left of the canvas.

        g.append("g")
         .attr("transform", "translate(0," + height + ")")
         .call(d3.axisBottom(xScale));
        
        g.append("g")
         .call(d3.axisLeft(yScale));

Step 7: Scatter dots

We now need to add a dot for every coordinate in our dataset1. We supply dataset1 to the data attribute and then make a circle for each coordinate. The cx specifies the horizontal position of the circle, while cy specifies the vertical position of the circle. Moreover, r specifies the radius of the circle. Then, we translate all the data points to match the translation of our axes. Lastly, we color the data points red by giving the #CC0000 hex-code:

        svg.append('g')
        .selectAll("dot")
        .data(dataset1)
        .enter()
        .append("circle")
        .attr("cx", function (d) { return xScale(d[0]); } )
        .attr("cy", function (d) { return yScale(d[1]); } )
        .attr("r", 2)
        .attr("transform", "translate(" + 100 + "," + 100 + ")")
        .style("fill", "#CC0000");

Step 8: Plot Line

Finally, we need to connect all the dots we added in step 8. To make a line, we will use the line generator of d3 by invoking d3.line(). This line generator can then be used to compute the d attribute of an SVG path element. We append path to our SVG and then specify the class as line. Then we transform it like we transformed our axes and datapoints earlier, and lastly, we style the line accordingly.

var line = d3.line()
        .x(function(d) { return xScale(d[0]); }) 
        .y(function(d) { return yScale(d[1]); }) 
        .curve(d3.curveMonotoneX)
        
        svg.append("path")
        .datum(dataset1) 
        .attr("class", "line") 
        .attr("transform", "translate(" + 100 + "," + 100 + ")")
        .attr("d", line)
        .style("fill", "none")
        .style("stroke", "#CC0000")
        .style("stroke-width", "2");

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