How can polygons be cosidered a subcategory of two dimensional figures?

Because they lie on a flat plane and are bounded by straight lines.

The major difference is that a histogram is only used to plot the frequency of score occurrences in a continuous data set that has been divided into classes, called bins.  bar charts, on the other hand, can be used for a great deal of other types ofvariables  including  ordinal  andnominal  data sets.