Midpoints of the interval of corresponding rectangle in a histogram are joined together by straight lines. It gives a polygon i.e. a figure with many angles. It is used when two or more sets of data are to be illustrated on the same diagram such as death rates in smokers and non-smokers, birth and death rates of a population etc. One way to form a frequency polygon is to connect the midpoints at the top of the bars of a histogram with line segments (or a smooth curve). Of course the midpoints themselves could easily be plotted without the histogram and be joined by line segments. Sometimes it is beneficial to show the histogram and frequency polygon together.But sometimes, the frequency polygon is much more accurate than the histogram because you can evaluate which is the low point and the high point. Unlike histograms, frequency polygons can be superimposed so as to compare several frequency distributions. Frequency polygons were created in the 19th Century as a way of not only storing data[citation needed], but making it easily accessible for people who are illiterate[citation needed]. Pearson, Karl. "Contributions to the mathematical theory of evolution. II. Skew variation in homogeneous material." Philosophical Transactions of the Royal Society of London 186.Part I (1895): 343-424.
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