Bi- Variate Map: This type of map or choropleth includes two variables on a map representation. It enables us to portray two separate phenomena simultaneously. The two variables should be related to each other as the bivariate map will show agreement or agreement between the variables. If you do not expect any association between them, then a bivariate map is not the right choice.
One of the most important features of choropleth is that it represents only normalized data: standard deviations, nested means, quantiles, and equal areas.
This infographic visualization piqued my interest in bivariate maps and how and where they should be used. Here it is a bivariate map as it renders size by frequency and color by severity simultaneously.
Due to the overlap of data over a particular data point, the different variates are not visible and the saturation and intensity of the color are lost.
To solve this problem, hexagonal binning has been used which is an effective way to aggregate and visualize data. Binning here represents the number of points that fall within a hexagon on the gridded map.
The dots are proportionally sized by the amount of time over the past five years that experienced drought (the largest dots representing 80% – 100% of the time). It is difficult to show time as a dimension in a static map and is shown in this map by representing each location by how much it has experienced any drought over the past five years. (0-20%, 20-40%, 40-60%, 60-80%, 80-100%).
The second variate is the color which is a weighted value of the intensity of those droughts (deep purple indicates frequent “exceptional” most severe droughts). It is based on the weighted sum of the number of weeks that it experienced droughts(worse droughts count more).
Both the variates are joined on the map by location and that makes it easy to understand. It is one of the most effective ways to show the dynamics of drought and can be used to represent social, geographic and demographic data in a more potent visualization form.
Source: https://adventuresinmapping.files.wordpress.com/2016/07/fiveyearsofdrought1.jpg