The histogram is only available for Pro members.
What is the histogram for?
As a Pro member, when you open contrast, you will notice a graph at the top of the window. That’s the histogram. It shows you the RGB (red, green, and blue) values of your image. The values at the extreme edges of the histogram represent the shadows (left) and highlights (right).
For images that are a little too light and washed out, move the left contrast slider to the right to darken shadows and bring out more detail.
For images that are too dark, move the right contrast slider to the left to increase highlights and reveal detail and color.
Values in the middle of the histogram represent the various ranges of color. Moving the brightness slider helps to increase the range of colors you see and can unveil more detail. And use the contrast sliders to continue improving the image.
The goal here is to pull out more detail by reducing the extreme shadows and highlights, so you really start to see the subtle variations of color, bringing details into view.
Use the RGB drop-down menu below it to change what information is displayed. Select the color you want to see, then use the sliders to increase or decrease the presence of that color.