Today at Flourish we’re launching a Heatmap template, the latest in our growing library of easy-to-use templates for data visualization and storytelling. Heatmaps are great for revealing patterns across two or three numeric or categorical variables.
The template was inspired by the climate stripes visualization, a “1D” heatmap by climatologist Ed Hawkins showing global temperature rise since the nineteenth century. You can now create this iconic visualization in Flourish, with interaction!
The template also supports more traditional 2D heatmaps, such as the following example by the Estonian Statistical Office, which reveals a clear negative trend in a particular month across all industrial sectors.
Since this is Flourish, the template also provides interactive features, such as a filter to let the viewer explore different categories or subsets of the data. You can even add a flip or fade animation when the data changes, as shown in the example below when you click the dropdown.
To make a heatmap, you just need a spreadsheet with need one column of data for each axis, plus another to shade by. All the examples above have categorical columns (or dates being treated as categories!) for the axes, and a numeric column for shading.
This isn’t fixed, however. You can also plot numeric columns on one or both axes, as shown in the first example below. To do this set “Numbers” under “Data type”, and the template will divide the data into “bins”, each representing an equal range along the axis. Conversely, it’s possible to use a categorical column for coloring, as shown by in the second example below.
When shading by a numeric variable, the template supports sum aggregation, meaning that if there are multiple rows of data represented in each cell, the shading will be based on their total value.
As in other Flourish templates, rich settings are available to control numeric color scales. Try a quantile scale to create color bins that each contain an equal number of cells on the heatmap, for example, or try HSL mode to give more vibrant colors.
Like other Flourish visuals, the Heatmap template is fully responsive for mobile. It also supports various sizing modes. You can let the cells resize to fill the available space or you can fix their aspect ratio (e.g. to make little squares) or their height. The latter option works well with lots of categories on the Y axis, since the label font size and cell height scale in parallel across screen sizes. A nice example of this is the example below by Sky News. This was published in their story about congestion levels during the coronavirus crisis and shows the stark drop-off of congestion levels in UK cities during lockdown.