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A color-magnitude diagram (CMD or CM) is a graph plotting color index against magnitude. Plotting color and (possibly) apparent magnitude shows some of the variety and patterns of the individual astronomical objects using observational data, i.e., bypassing some analysis that can be imperfect. Such a diagram plotting individual stars is basically a type of H-R diagram (HRD): color indexes depend upon temperature and absolute magnitude is luminosity on a log scale. The stars of a single stellar cluster can usefully be plotted using apparent magnitude, for comparison with each other and discernment of overall patterns. Galaxies are often plotted using absolute magnitude (galaxy color-magnitude diagram), which reflects their mass and their color is some indication of star formation: the distinction is imperfect (massive stars add to the mass and affect the color) but again, the color-magnitude diagram bypasses some of the analysis necessary to create the common alternative: mass versus star formation rate galaxy plots.
A Hardness-Intensity Diagram (HID) is an X-ray variant on the concept: hardness generally short for hardness ratio, which is a color index within the X-ray spectral band, and intensity measured as photons received by an instrument during an observation.