Travis Horn
Travis Horn

Travis Horn

Build a Sparkline Vue Component

Build a Sparkline Vue Component

Travis Horn's photo
Travis Horn

Published on Aug 15, 2020

7 min read

Sparklines can be used to quickly visualize data variance. They are small and intuitive to understand.

We’ll be using Vue and D3 to build a small sparkline component using Vue.js. In the next article, I’ll show you how to use this component to build a “dashboard” for easy visualization of many data points.

First, make sure you have the Vue CLI installed. Then, create a new project.

vue create sparkboard

The CLI will ask you questions about how you’d like to set up your project. You can accept the defaults or manually select features you like. I manually selected…

  • Babel

  • Linter/Formatter

  • ESLint + Prettier

  • Lint on save

  • In dedicated config files

Once the project has been created, start the dev server.

cd sparkboard
npm run serve

Create a new component in the src/components directory called SparkLine.vue

<template>
  <svg viewBox="0 0 200 40">
    <path d="M 0 20 H 200"
          fill="none"
          stroke-width="3"
          stroke-color="gold" />
  </svg>
</template>

<script>
export default {
  name: "SparkLine"
}
</script>

Notice the root element of the component is an &lt;svg&gt;. This is the element we’ll bind D3 to when creating the chart. You can also see that the &lt;svg&gt; has a viewBox of 0 0 200 40. This gives it the dimensions of 200 pixels wide by 40 pixels tall. The actual width and height are determined by the element’s container, but the 200 x 40 viewBox gives it a good height to width ratio for a sparkline chart.

Finally, there is a &lt;path&gt; inside the &lt;svg&gt;. This is our actual line. For right now, it’s just a hard-coded, perfectly straight, horizontal line.

We can see our progress so far by putting our component in the main App.vue. In that file, replace everything in the template with…

<template>
  <div style="width: 100px">
    <SparkLine />
  </div>
</template>

Notice how we put the sparkline component inside a container element with a narrow width. This will give it the traditional small appearance.

Now, replace the script section with…

<script>
import SparkLine from "./components/SparkLine.vue";

export default {
  name: "App",
  components: {
    SparkLine
  }
};
</script>

You may notice how we just changed HelloWorld to SparkLine. Everything else is the same.

Since we cleared out the template, we no longer need the default styles. So feel free to remove everything from the &lt;style&gt; section.

Now run the development server.

npm run serve

With the server running, we can visit http://localhost:8080/ and see our progress so far.

It is literally just a horizontal yellow line, which is perfect because that’s exactly what we’ve coded so far.

Let’s code a way to pass data into this component so we can display it using this line.

Going back to SparkLine.vue and adding to the script section, define a prop called data. This prop will be an array. We’ll also tell it to simply use a basic array of [0, 0] if no data is passed in.

<script>
export default {
  name: "SparkLine",
  props: {
    data: {
      type: Array,
      default() {
        return [0, 0];
      }
    }
  }
};
</script>

We can pass data into the component back in App.vue like so:

<SparkLine :data="[808, 1475, 1426, 1884, 1396]" />

Note that the line above replaces the existing &lt;SparkLine /&gt;. Also note that these are just random values I made up. Feel free to use whatever you like.

Now it’s time to use D3.js to bind data to the line. First, install D3.

npm i d3

Then, in SparkLine.vue, import it at the top of the &lt;script&gt; section.

import * as d3 from "d3";

When the component is mounted, we need to tell D3 to set up the chart. So below the props definition, add a mounted() function.

export default {
  name: "SparkLine",
  props: { /* ... snip ... */ },
  mounted() {
    // We will set up the chart here
  }
};

Inside the mounted() function, tell D3 where our chart will go.

this.chart = d3.select("svg"); // Targets the (only) SVG element

Now we need to set up x and y scales. The x axis will start at the very start (0) of our chart and end at the very end (right) of our chart (200).

this.x = d3.scaleLinear().range([0, 200]);

And the y axis will start at the bottom (40) and end at the top (0) of our chart.

this.y = d3.scaleLinear().range([40, 0]);

If you’ve used D3 scales before, you might be wondering why we’re not setting the domain here. That’s because we’re going to put that logic in a separate function that can get called any time our data changes.

Now we must define the line function.

this.line = d3
  .line()
  .x((d, i) => this.x(i))
  .y(d => this.y(d));

What the code above does, is defines a D3 line where the x value is determined by the position of the data in the array and the y value is determined by the data point itself.

Finally, let’s call a method to actually plot the data.

this.plot();

The function called in the line above doesn’t exist, yet. Let’s create it now.

This will go in our components methods.

export default {
  name: "SparkLine",
  props: { /* ... snip ... */ },
  methods: {
    plot() {
      // Plotting code goes here
    }
  },
  mounted() { /* ... snip ... /* }
};

The first thing to do inside of plot() is to set the domain of our scales according to the data.

this.x.domain([0, this.data.length - 1]);
this.y.domain(d3.extent(this.data));

As you can see above, the x scale’s domain starts at 0 and ends at the end of the array. The y scale’s domain starts with the smallest data value and ends with the largest.

Now to actually adjust the chart line. Also in plot():

this.chart.select("path").attr("d", this.line(this.data));

If you save SparkLine.vue and look again at http://localhost:8080/ (assuming the development server is still running), you can see the sparkline!

What happens if our data changes? All we need to do is call the plot() method again. Set up a watcher to do that. This watcher will “watch” the data property and call plot() any time it changes.

export default {
  name: "SparkLine",
  props: { /* ... snip ... */ },
  watch: {
    data() {
      this.plot();
    }
  },
  methods: { /* ... snip ... /* },
  mounted() { /* ... snip ... /* }
};

And now our component can react to data changes!

While all of this works as a single component, there’s one important bug that we have to fix if we want to re-use this component more than once on the same page. Right now, we have D3 selecting the sole &lt;svg&gt; element on the page. If you have more than one &lt;svg&gt; on the same page, D3 is just going to select the first one it finds. This is no good. We want to select the &lt;svg&gt; in this component.

To do that, we can assign our &lt;svg&gt; a unique ID.

First, install nanoid.

npm i nanoid

Import it into our component at the top of the &lt;script&gt; section.

import { nanoid } from "nanoid";

Create a new id data property and use nanoid to make it random.

export default {
  name: "SparkLine",
  props: { /* ... snip ... /* },
  data() {
    return {
      id: `chart-${nanoid()}`
    };
  },
  methods: { /* ... snip ... /*},
  mounted() { /* ... snip ... /*}
};

We use the template string chart-${nanoid()} so that our random id is in the format chart-kHM_8K1yz8GGq6MVBwfoG or chart-VG5J13fYZIdMBrSDRuEBX for example. It’s always random.

In the template, set the &lt;svg&gt;'s id to this new id.

<svg :id="id" viewBox="0 0 200 40">

Finally, tell D3 to select the element based on this ID.

Replace

this.chart = d3.select("svg");

with

this.chart = d3.select(`#${this.id}`);

That’s it Our sparkline component is done! You can add multiple sparkline components on the same page and they’ll all show up perfectly.

<template>
  <div style="width:100px">
    <SparkLine :data="[808, 1475, 1426, 1884, 1396]" />
    <SparkLine :data="[3246, 1941, 2649, 1633, 1262]" />
    <SparkLine :data="[190, 128, 209, 208, 116]" />
  </div>
</template>

All code for this article is open source and published on GitHub. travishorn/vue-sparkline-component Basic sparkline component for Vue.js using D3.js npm install npm run serve npm run build npm run lint See Configuration…github.com

In the next article, I’ll show you how to use this component to build a “dashboard” for easy visualization of many data points.

Exercise to the reader

The component is hard-coded to have a yellow line. What if you wanted to be able to set the color individually per component? Hint: use a prop (and don’t forget to set a default value!).

 
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