Theory:

Frequency polygon
A frequency polygon is a line graph, and it is obtained by joining the mid-points of the frequency rectangles.
Let us look at an example of a frequency polygon.
 
1276_6.svg
 
The line graph forms a closed surface with many sides, hence the name frequency 'polygon'.
Types of frequency polygon
Frequency polygons are of two types, namely:
 
1. Frequency polygon with histogram
 
2. Frequency polygon without histogram.
 
When the line graph is over a histogram, then it is a frequency polygon with a histogram.
 
When the line graph alone is visible while the histogram is not, it is a frequency polygon without a histogram.