Histogram

A histogram is a two-dimensional diagram made of rectangles: the base of each rectangle is a class interval (marked between class boundaries on the X-axis) and its area is proportional to the class frequency.

When all class intervals are of equal width, the areas — and hence the heights — are simply proportional to the frequencies, so the height of each rectangle can be taken as the frequency itself. When the widths are unequal, the heights must be adjusted so the areas stay comparable; the height then used is the frequency density:

$\text{frequency density} = \frac{\text{class frequency}}{\text{width of the class interval}}.$ …