The purpose of these heatmap visualizations is to show users at a glance when categories deviate from the growth rate which would be expected if each individual category were to follow the rate of the Thesaurus as a whole. The heatmaps were designed to investigate category behaviour in the Early Modern period and as such display data for the decades between 1470 and 1800. As with the sparkline visualizations, heatmaps employ the human-scale thematic category set. Green cells indicate that a category is larger than would be expected in a particular decade, blue cells indicate that a category is smaller than would be expected; the darker the colour of a cell the further it is from the expected norm, the lighter it is, the closer it adheres to the standard growth of the Thesaurus.

The underlying data contains counts of all the active word senses in the Thesaurus per decade. These are defined as those for which citation evidence attests that the sense was in use during the time period in question; as well as excluding senses which were not coined within the given period, a count of active senses excludes any which fell out of use before that period began. For each decade, a ratio is then calculated of the number of senses active in that decade to the category’s ‘peak’ count (i.e. the decade in which the most senses are active). Similar ratios are then calculated for each of the thematic categories, expressing the size of the category in each decade as a proportion of that category’s maximum size. Finally, the Thesaurus’ content ratio is subtracted from the category content ratio to give the difference between the size of the category in a decade and the size of the Thesaurus in the same decade.

This numerical data is represented as coloured cells on the heatmap, showing whether a category is larger or smaller than it would be expected to be in each decade if it were following the growth pattern of the entire Thesaurus. Each decade in a category’s history is represented as a cell on the heatmap. Owing to the large number of categories in the thematic category set, users select a top level thematic category from a drop down menu, which will display not only the selected category but all of its daughter categories.

Hovering over a cell in the heatmap, tooltip information boxes appear giving the decade represented by that cell and the value for the difference in size between the category’s contents and the Thesaurus contents. Because the contents of each cell are the difference between two ratios, the values are always between -1 and 1. The more positive a value is, the more it indicates that the category is larger in this decade than would be expected if it were to follow the growth pattern of the full Thesaurus. If a value is negative, it suggests that the category is smaller than it would be expected to be.

These measurements of comparative size indicate deviation from the normal rate of growth for the Thesaurus; positive values can therefore be interpreted as faster category growth than would be expected, and negative values as slower growth than expected. Large positive values for a category’s proportion of its total compared to the Thesaurus’ proportion of its total indicate a period of rapid growth of this category.