http://www.davidmlane.com/hyperstat/A49236.html

bar graphs

http://cstl.syr.edu/FIPSE/TabBar/RevBar/REVBAR.HTM

Frequency Polygon

A frequency Polygon is a graphical representation of data in which the mid – values of the class intervals of the given data is plotted against the corresponding frequencies and the points so obtained are joined by straight lines.
Follow these steps to draw a frequency Polygon
Represent the class marks i.e., mid points of class intervals along the x – axis and the frequency along the y – axis.
Plot the points corresponding to the frequency at each class mark.
Join these points, in order, by straight lines.
To complete the polygon, join the point at each and immediately to the lower or higher class mark as the case may be at 0 frequency i.e., on the x- axis.
Let us consider an example
Ina study of diabetic patients, the following data was obtained
There are 3 diabetic patients, in the age group of 10 to 20 years.
6 in the age group of 20 to 30 years
10 in the age group of 30 to 40 years
9 in the age group of 40 to 50 years
5 in the age group of 50 to 60 years
2 in the age group of 60 to 70 years
Represent this data by a frequency polygon.
The table gives the class marks and the frequency
For, example, for the age group 10 – 20 years, the class mark is 10 plus 20 by 2 is equal to 15.
The frequency is the same as the no. of diabetic patients in that age – group, i.e., 3.
In the graph below x axis represents the age in years while y axis represents the frequency.
Hence, the data in the table can be represented by the points (15,3), (25,6), (35,14), (45,9), (55,5), (65,2), and lastly (5,0), and (75,0). Mark these points on the graph. Join the points in the order.
This gives the frequencies polygon for the given data