statistics:
    A frequency polygon is a line graph of a frequency distribution. The classes are measured along the horizontal axis and frequencies along the vertical axis.
    Each endpoint of each line is plotted at the class midpoint. First line is connecting first class and second class, second line is connecting second and third class, ...
    
    For example:
    10 students received following grades from last test: 1, 2, 4, 1, 5, 2, 5, 3, 1, 5. The grades are our data and it can be divided into 5 classes: 1, 2, 3, 4, 5.
    Or by the frequency polygon (red):