RdBu

RdBu is a diverging color map widely used in data visualization to encode values that diverge from a central reference, such as zero or the mean. In its standard form, the scale runs from blue on the low end, through a neutral midpoint, to red on the high end, making it suitable for emphasizing positive and negative deviations from the center. A reversed variant, RdBu_r, inverts the color mapping so high values appear blue and low values appear red.

Origin and availability: The naming follows ColorBrewer palettes, developed by Cynthia Brewer, and the RdBu palette has become a common choice in many plotting libraries. It is implemented in Matplotlib as 'RdBu' (and 'RdBu_r'), in seaborn, R packages, and in D3, among others. It is often used for heatmaps, choropleth maps, and anomaly plots where a neutral midpoint is important.

Usage guidance: When using RdBu, ensure the data are symmetric around the midpoint and that the color scale provides perceptual uniformity. For readers with color vision deficiencies, consider testing with colorblind-friendly palettes or adding patterns or labels. In high-stakes mapping, provide a legend indicating the midpoint and ensure contrast is sufficient for black-and-white print.

Limitations: Diverging palettes like RdBu can be misleading if the data do not center naturally on a midpoint or if there are many identical values near the center. They should be used with appropriate normalization and clear legends.