A histogram is a type bar chart using intervals on the x axis, with the modification that the interval sizes are not fixed. The frequencies are defined by the areas of the bars, and the y – axis is not a frequency axis, but a frequency density axis. A histogram is better able to represent data because it can be used where the data is irregularly grouped. The heights of a group of fictitious children are summarised in the table below.
Height 
Frequency 

65 – 75 
22 

75 – 80 
7 

80 – 90 
21 

90 – 105 
15 

105 – 110 
12 
We construct a Frequency Density Column, usingobtaining the table
Height 
Frequency 
Frequency Density 
65 – 75 
2 
2/10 = 0.2 
75 – 80 
7 
7/5 = 1.4 
80 – 90 
21 
21/10 = 2.1 
90 – 105 
15 
15/15 = 1.0 
105 – 110 
12 
12/5 = 2.4 
We can now draw the histogram.