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Concept
Heat map
Summary
A heat map (or heatmap) is a 2-dimensional data visualization technique that represents the magnitude of individual values within a dataset as a color. The variation in color may be by hue or intensity.
"Heat map" is a relatively new term, but the practice of shading matrices has existed for over a century.
Heat maps originated in 2D displays of the values in a data matrix. Larger values were represented by small dark gray or black squares (pixels) and smaller values by lighter squares. Toussaint Loua (1873) used a shading matrix to visualize social statistics across the districts of Paris. Sneath (1957) displayed the results of a cluster analysis by permuting the rows and the columns of a matrix to place similar values near each other according to the clustering. Jacques Bertin used a similar representation to display data that conformed to a Guttman scale. The idea for joining cluster trees to the rows and columns of the data matrix originated with Robert Ling in 1973. Ling used overstruck printer characters to represent different shades of gray, one character-width per pixel. Leland Wilkinson developed the first computer program in 1994 (SYSTAT) to produce cluster heat maps with high-resolution color graphics. The Eisen et al. display shown in the figure is a replication of the earlier SYSTAT design.
Software designer Cormac Kinney trademarked the term 'heat map' in 1991 to describe a 2D display depicting financial market information. The company that acquired Kinney's invention in 2003 unintentionally allowed the trademark to lapse.
There are two main type of heat maps: spatial, and grid.
A spatial heat map displays the magnitude of a spatial phenomena as color, usually cast over a map. In the image labeled “Spatial Heat Map Example,” temperature is displayed by color range across a map of the world. Color ranges from blue (cold) to red (hot).
A grid heat map displays magnitude as color in a two-dimensional matrix, with each dimension representing a category of trait and the color representing the magnitude of some measurement on the combined traits from each of the two categories.
Official source
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