Colz

Colz is a drawing option in the ROOT data analysis framework used to visualize two-dimensional histograms. It is commonly written as COLZ, with colz also accepted in many interfaces. The option renders a color-coded map of bin contents and displays a color bar showing the corresponding Z-axis scale. It is typically applied to TH2 and TH2F histograms by drawing the histogram with the COLZ option, such as h2->Draw("COLZ").

How it works: each bin in a 2D histogram is assigned a color based on its content, with higher values mapped to colors toward one end of the predefined palette. The resulting image provides a quick visual sense of density or event distribution across the two dimensions. ROOT offers multiple color palettes and allows users to customize the color map, including enabling or hiding the color legend.

Usage and options: COLZ can be used alone or combined with other display options, for example to overlay textual bin contents using the TEXT option. The behavior of the color mapping depends on the histogram’s data range and the chosen palette. ROOT’s drawing system treats COLZ as a rendering style and is case-insensitive in practice across interfaces.

Relation to other visualization options: COLZ contrasts with options that render as simple colored blocks without a color bar (such as COL) and with 3D or surface representations (such as SURF or MOVIE-like projections). COLZ is particularly favored when the goal is to emphasize the distribution of values across two variables rather than precise per-bin values.

See also: ROOT, 2D histograms, color palettes, data visualization in high-energy physics.