A heatmap is a graphical representation of statistical data where the individual values contained in a matrix are represented as colors. Fractal maps and tree maps both often use a similar system of color-coding to represent the values taken by a variable in a hierarchy.
There are multiple kinds of heat maps:
- Biology heat maps are typically used in molecular biology to represent the level of expression of many genes across a number of comparable samples as they are obtained from DNA microarrays or RNA-seqs.
- The tree map is a 2D hierarchical partitioning of data that visually resembles a heat map.
- Web heat maps have been used for displaying areas of a Web page most frequently scanned by visitors.
- A density function visualization is a heatmap for representing the density of dots in a map. It enables one to perceive density of points independently of the zoom factor.
- A mosaic plot is a tiled heatmap for representing a two-way or higher-way table of data. Like tree maps, the rectangular regions in a mosaic plot are hierarchically organized.
There are many different color schemes that can be applied to illustrate the heatmap, with perceptual advantages and disadvantages for each. Rainbow colormaps are usually applied, as humans can observe more shades of color than they can of gray, and this would purportedly increase the amount of detail perceivable in the image. But many researchers in the scientific community, for the following reasons, discourage it:
- The colors lack the natural perceptual ordering found in grayscale or blackbody spectrum colormaps.
- The changes between colors may lead to perception of gradients that aren't actually present, making actual gradients less protuberant.
- Common colormaps have uncontrolled changes in luminance that prevent meaningful conversion to grayscale for display or printing.
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