It is generally used for charting continuous frequency distribution. In histogram, data are plotted as a series of rectangle one over the other. Class intervals are shown on x axis and the frequencies on y axis. There are as many rectangle as there are classes. The height of each rectangle is taken equal to its frequency. It means the height of rectangle represent the frequency of that group. Each rectangle is gained with the other so as to give a continuous picture. The area of this graph represents the total frequencies .Such a graph is also known as block diagram or staircase diagram.