The Histogram   PSP ver. 7 Tutorial
Paint Shop Pro offers a wide range of tools to help you enhance your photos. Among these tools is the Histogram. To use it, simply click on the Histogram icon in the Toolbar menu. The Histogram window will open and display in graphical form the selected information in your image. This can be the entire image or areas that you have selected within the image.
As an example, let's look at the image below and see what the histogram tells us about it.
The graph is stretched pretty much over the entire lightness range. This tells us that the picture is balanced and probabbly has enough detail to allow for corrections. How do I know that? Well here are some simple rules.

If the graph is spread out, the image has a balanced composition and enough detail to make corrections to it.
If the lines are compressed into a narrow range you probably need to rescan the image or take another picture.
If the grayscale graph is mostly on the left, the image is too dark and you will need to increase the image's lightness.
If the grayscale graph is mostly on the right, your image is too ligt and you will need to decrease the image's lightness or make it darker
If the grayscale lines are not spread out enough, you may need to stretch the Histogram or increase the contrast.
The check boxes below the graph selectively enable the channels Red, Green, Blue, Grayscale, Hue, Saturation and Ligthness. Checking and unchecking each of the boxes will turn the respective graph on and off for you to examine.
The CheckBoxes
To the right of the channel check boxes are the Sample merged and Selection Only boxes.

When Sample Merged is checked, the information displayed by the graph will include all the layers in your image.
When Selection Only is checked, the information displayed by the graph will represent oly the area selected in your image.
Now let's look at anothe image. The Histogram looks strange at best, but the image can still be improved.
Look at the tutorial The Histogram Adjustment Tool to see how both images were improved.