Histograms
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  2 

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.