Small Multiples on River Quality

The phrase small multiple was popularised by Edward Tufte, and has become a generic term for a visual display using the same chart or graphic to display different slices of a data set. Their close positioning and shared scale make comparisons very easy and shared trends or outliers can be quickly spotted. Various other terms are also used to describe this charting approach, or specific aspects of it, including Trellis Charts, Lattice Charts, Grid Charts and Panel Charts.

The most common use case for small multiples is separate line charts to compare trend across a large number of varying elements. Placing them all within one chart would cause either a ‘spaghetti chart’ , or lots of occlusion as shown in the comparison below. Here we use a standard Excel line chart, and an XLCubed small multiple to chart the same data. Separating the charts while keeping a consistent axis scale makes for a much easier comparison than in the single chart.

We took a slightly different approach when using small multiples to take a look at differences in river water quality across regions of the UK. Our source data was not absolute numeric values, but 14 years of results categorised into four bandings (bad, poor, fair and good). We wanted to provide a ‘one-pager’ which gave a feel for the trend within each region, but also access to the annual breakdown of the different water qualities.

In the end we settled on a Small Multiple display of 100% stacked columns as shown below.

A percentage base seemed a sensible way to approach the data, as different regions will have differing numbers of rivers and of samples taken. Using this approach we’re able to see a comparison of the relative water quality rather than dealing in absolutes.

The user selects a geographic area of the country to view the regional breakdown within the selected area. The water quality for a particular year can be analysed by locating the region, and the specific year to see the percentage breakdown for each of the four categories.

The colouring of the 4 categories was chosen to aid ‘at a glance’ recognition of the overall water quality by region, and also of the trend. Dark blue signifies bad quality water (opaque), and light blue signifies good quality (think ‘you can see right through it….’).

So to read the display overall, or for trend:
• Dark colour signifies water quality problems.
• Light colour signifies good quality water.
• Reading left to right, increasing colour saturation shows declining quality over time.
• Reading left to right, decreasing colour saturation shows improving quality over time.
• Any region can be zoomed in on to see a larger chart and understand the breakdown in more detail.

Fairly quickly, and from just this one display we can draw a number of conclusions as below:
• Across the region, as a broad brush summary, water quality has improved since 1992.
• Doncaster has shown strong and steady improvement.
• Kingston upon Hull has the worst quality overall in the region, and varies significantly year on year.
• If you’re off for a swim in a Yorkshire river, Richmondshire looks a good bet!

We’ve designed a pre-set view in this case to work for the data in question, but the small multiple concept is also very powerful when interactively exploring data. A picture can tell a thousand words as they say – take a look at our youtube videos on small multiples: Video1 Video2

 

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