This is part of a series of posts about the 'little of visualisation design', respecting the small decisions that make a big difference towards the good and bad of this discipline. In each post I'm going to focus on just one small matter - a singular good or bad design choice - as demonstrated by a sample project. Each project may have many effective and ineffective aspects, but I'm just commenting on one.
The 'little' of this next design concerns the referencing of spatial information. This graphic, by the Bloomberg Visual Data team, shows trends of the growing number of unfracked wells across the different basins of the US. When one has quantitative data associated with geographic locations there is always a temptation to pursue ways of creating a thematic map display, representing the data onto and within a map structure.
However, thematically mapping data is only of value if the most interesting qualities of the subject lie in the potential patterns across and relationships between the regions in question. In this instance, the interesting angle is more about the comparison of quantities over time across all the different 'categories' of basin locations NOT the comparison between basin locations themselves. Therefore, the line charts are the most appropriate way to represent the data and the role of the map switches to being a visual reference for the basin locations, from which the respective charts branch off