The vertical axis of histogram can be . The frequency shows the number of observations within a band. When "Density" is selected from the drop-down menu, areas for each bar in the histogram represents the probability that an observation falls in the range specified by a bar. For this reason the density height must be calculated by the formula:
The width of each class (or band) is called class width. If the class width is too large then the graph fails to convey the structure of the data. On the other hand if it is too small then it becomes too spiky.
If the interval between and is also specified, it calculates the frequency (or the proportion) of the observed values within the interval [a,b). (The interval will be [a,b] if is the maximum of data.) With the density height at the vertical axis, the area under the histgram in the interval [a,b) represents the proportion of the sample distribution.