Briefly |
A density axis (usually in a histogram) shows how much of the data there is per unit of measure on the other axis. |
Usually we see histograms with a vertical axis scaled in counts, that is, the bars show how many cases are in each bin. But real official histograms (and Ntigrams) have a density axis instead.
A density axis for a histogram is scaled in units of proportion per <whatever>, where <whatever> is the units on the other axis. For example, on the left is a histogram of age:
We can see that, for example, there are about 38 kids between ages 0 and 5. Since there are 500 cases altogether, that amounts to about 7.5% of the total. On the right is the same graph with Use Density Scale turned on (in the Graph menu). There, the same bin has a height of about 0.015--one and a half percent. That's the percentage per year of age. Since there are five years in the bin, that makes 7.5% altogether.
The axis in an Ntigram is slightly different--it's in cases per year instead of proportion per year:
As you can see, the Ntigram has different bin widths.