Heat Maps

Abstract

In 1873, Toussaint Loua created a heat map. A heat map is a matrix of color-coded cells for a response variable, typically numeric, using X and Y variables to label the columns and rows. If the X and/or Y data is continuous numeric, those values will be “binned.” The full range of the values is divided into subranges or bins. In that case, the bins, when identified by axis values, are represented by midpoints of the subranges, if ODS Graphics presents the axis values, or by the average of the variables’ values within each subrange if using the custom macro provided in this book. For binned X and Y pairs, the numeric response value for each cell in the matrix can only be some statistic (average, sum, frequency, etc.) for observations with that pair of X and Y values. See examples in Figure 6-1.