Relative frequencies of class intervals can also be shown in a frequency polygon. In this chart, the frequency of each class is indicated by points or dots drawn at the midpoints of each class interval. Those points are then connected by straight lines.
The frequency polygon shown in Figure 1 uses points, rather than the bars you would find in a frequency histogram.
Figure 1
Figure 1 Frequency polygon display of items sold at a garage sale.
Whether to use bar charts or histograms depends on the data. For example, you may have qualitative data—numerical information about categories that vary significantly in kind. For instance, gender (male or female), types of automobile owned (sedan, sports car, pickup truck, van, and so forth), and religious affiliations (Christian, Jewish, Moslem, and so forth) are all qualitative data. On the other hand, quantitative data can be measured in amounts: age in years, annual salaries, inches of rainfall. Typically, qualitative data are better displayed in bar charts, quantitative data in histograms. As you will see, histograms play an extremely important role in statistics.
Hope the above explanation was useful.
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