A Histogram is enticing because it exhibits a clear relationship between the brightness of the image and the configuration of the graph. Make the image brighter, and the graph moves generally to the right which reads as “more” to people used to reading written language left to right. Histograms are used to visualise all sorts of data; a graph indicating, say, the average cake slice size favoured by various age groups might also be a Histogram. A Histogram is a distribution graph, in which the horizontal axis represents luminance (or brightness) and the vertical axis represents the proportion of pixels in the image which are at that particular luminance. A brightly lit subject of a light colour might contain pixels which are mainly above 50% of maximum luminance, which will create a fuller histogram on the right hand side, toward the high-luminance end of the horizontal axis. Conversely, an image containing mainly black or low-luminance pixels will fill up the graph on the left-hand, lower-luminance end.
The test image as viewed on a histogram. The image is predominantly dark, so the histogram is weighted heavily to the left