CSS Slope Graphs

2021-06-07

I am a huge sucker for simplistic and beautifully designed visual data on the web. Most data tends to be graphed via line or bar systems - which is fine - but I think slope graphs are highly underrated. Let’s change that, shall we?

The Demo

I’m basing this demo off the design patterns found in Edward Tufte’s visualization work, specifically his slope graph designs:

Live CodePen Example

The HTML

For this concept we will actually be building this graph out of tables - crazy, right? The greatest benefit of rendering all the data inside of a table element is the ability to easily support smaller screens and mobile devices. Larger viewports will get to see the pretty slope graph, while those below a certain threshold will view a simple table.

(But more on that in the CSS section)

<p>Sales of the leading frozen pizza brands of the United States from 2011 to 2017 (in million US dollars) <br><em>Source: Statisa 2018</em></p>
<table>
  <thead>
    <tr>
      <th>Pizza Brand</th>
      <th>2011</th>
      <th>2017</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td data-set="677.0">DiGiorno</td>
      <td><span>677.0</span></td>
      <td data-name="DiGiorno">1014.6</td>
    </tr>
    <tr>
      <td data-set="294.8">Private Label</td>
      <td><span>294.8</span></td>
      <td data-name="Private Label">524.8</td>
    </tr>
    <tr>
      <td data-set="286.1">Red Baron</td>
      <td><span>286.1</span></td>
      <td data-name="Red Baron">572.3</td>
    </tr>
    <tr>
      <td data-set="257.9">Tombstone</td>
      <td><span>257.9</span></td>
      <td data-name="Tombstone">270.6</td>
    </tr>
    <tr>
      <td data-set="164.5">Totino's Party Pizza</td>
      <td><span>164.5</span></td>
      <td data-name="Totino's Party Pizza">347.2</td>
    </tr>
  </tbody>
</table>

As you can see, nothing too fancy is happpening here. Pay close attention to the data-set and data-name variables though - those will be important for the CSS portion of this design, mainly the rendering of the line elements.

The CSS

To avoid overwhelming your brain all-at-once, let’s break the CSS down into bite-sized chunks, starting with the base styling:

@import url('https://opentype.netlify.com/et-book/index.css');
* {
  box-sizing: border-box;
}

html {
  height: 100%;
}

body {
  background: #fffff8;
  font-family: "et-book", serif;
  height: 100%;
  margin: 0 auto;
  max-width: 800px;
  padding: 0 0.5rem;
}

p {
  font-size: 18px;
  margin: 4rem 0 6rem;
}

table {
  border-collapse: collapse;
  text-align: left;
  width: 100%;
}

Pretty basic stuff.

Now we need to design how our slope graph will look on larger screens / desktops. For this instance, we will target these larger devices with a min-width media query of 800px. The rest of the CSS might look a little confusing but I assure you it is quite simple.

1. On larger devices we hide the first thead tr th element with display: none
2. The first and second td elements inside each tbody row need to be set as position: absolute to avoid duplicate content
3. The inner span that we include in our HTML inside the second tbody tr td also needs to be display: none
4. Remember that data-set variable? We now use that for our :before pseudo element for table tbody tr td:nth-of-type(1)
5. Remember that data-name variable? We now use that for our :before pseudo element for table tbody tr td:nth-of-type(3)
6. After that, you can see the simple customization we include to render the angle / position of the slope lines and the corresponding labels

@media(min-width:800px) {
  table {
    display: block;
    position: relative;
    margin-bottom: 25rem;
  }

  table thead th {
    border-bottom: 1px solid lightgrey;
    font-size: 24px;
    position: absolute;
    top: -50px;
    width: 45%;
  }
  table thead th:nth-child(1){ display: none; }
  table thead th:nth-child(2){ left: 0; }
  table thead th:nth-child(3){ right: 0; text-align: right; }

  table tbody tr td:nth-of-type(1),
  table tbody tr td:nth-of-type(2) { position: absolute;}

  table tbody tr td:nth-of-type(2) span { display: none; }
  table tbody tr td:nth-of-type(1):before {
    content: attr(data-set);
    margin-right: 10px;
    position: relative;
  }

  table tbody tr td:nth-of-type(2) { padding-left: 10px; }

  table tbody tr td:nth-of-type(3) {
    position: absolute;
    right: 0;
  }
  table tbody tr td:nth-of-type(3):before {
    content: attr(data-name);
    margin-right: 10px;
    position: relative;
  }

  /* Custom individual slopes -- Left */
  tbody tr:nth-child(1) td:nth-child(1),
  tbody tr:nth-child(1) td:nth-child(2) { top: 60px; }
  tbody tr:nth-child(2) td:nth-child(1),
  tbody tr:nth-child(2) td:nth-child(2) { top: 140px; }
  tbody tr:nth-child(3) td:nth-child(1),
  tbody tr:nth-child(3) td:nth-child(2) { top: 165px; }
  tbody tr:nth-child(4) td:nth-child(1),
  tbody tr:nth-child(4) td:nth-child(2) { top: 220px; }
  tbody tr:nth-child(5) td:nth-child(1),
  tbody tr:nth-child(5) td:nth-child(2) { top: 270px; }

  /* Custom individual slopes -- Right */
  [data-name="DiGiorno"] { top: 0; }
  [data-name="Red Baron"] { top: 65px; }
  [data-name="Private Label"] { top: 100px; }
  [data-name="Tombstone"] { top: 180px; }
  [data-name="Totino's Party Pizza"] { top: 150px; }

  /* The custom visual lines */
  tbody tr:after {
    background: black;
    content: '';
    height: 1px;
    left: 14.5%;
    position: absolute;
    width: 70%;
  }
  tbody tr:nth-child(1):after {
    top: 40px;
    transform: rotate(-6deg);
  }
  tbody tr:nth-child(2):after {
    left: 17.5%;
    top: 130px;
    transform: rotate(-4deg);
    width: 65%;
  }
  tbody tr:nth-child(3):after {
    left: 15%;
    top: 125px;
    transform: rotate(-10.25deg);
    width: 70%;
  }
  tbody tr:nth-child(4):after {
    left: 16%;
    top: 210px;
    transform: rotate(-4deg);
    width: 68%;
  }
  tbody tr:nth-child(5):after {
    left: 22%;
    top: 222px;
    transform: rotate(-16deg);
    width: 56%;
  }
}

All that’s left are some minor styles to make everything look nice on mobile:

@media(max-width:800px) {
  p {
    margin: 2rem 0;
  }
  table td, table th {
    border-bottom: 1px solid grey;
    padding: 10px;
  }
  table td:last-of-type, table th:last-of-type {
    text-align: right;
  }
}

Not the most practical

This slope graph concept is far from perfect for use in real-world situations. The fact that you need to manually render each point of data yourself makes this implementation quite annoying for more in-depth projects.

But it was fun to mess around with and create, so who cares!