# 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](https://www.edwardtufte.com/tufte/books_vdqi), specifically his slope graph designs:

[Live CodePen Example](https://codepen.io/bradleytaunt/pen/jOBzXMe)

## 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!