A histogram is a graph of the distribution of RGB values in an image. Each of the four values appear on the horizontal axis from dark to light (left to right). The vertical axis indicates the number of pixels of that value at each point. At a point where there are many pixels of a value, the corresponding line spikes; where there are no pixels, it lies at the bottom of the graph.

First I used colourize feature to increase image saturation and hue, then applied a ore-defined history equalize feature. It looks like an old photo becoming yellowish.