Heat map

A heat map (or heatmap) is a graphical representation of data where the individual values contained in a matrix are represented as colors. 'Heat map' is a newer term but shading matrices have existed for over a century.Lake effect snow – weather radar information is usually shown using a heat map.Human voice visualized with a spectrogram; a heat map representing the magnitude of the STFT. An alternative visualization is the waterfall plot.Example showing the relationships between a heat map, surface plot, and contour lines of the same dataCombination of surface plot and heat map, where the surface height represents the amplitude of the function, and the color represents the phase angle.Score of each contiguous region of a dartboard (not to scale) A heat map (or heatmap) is a graphical representation of data where the individual values contained in a matrix are represented as colors. 'Heat map' is a newer term but shading matrices have 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. 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.