This is called a frequency polygon.

In a Frequency Polygon, a line graph is drawn by joining all the midpoints of the top of the bars of a histogram. A frequency polygon gives the idea about the shape of the data distribution. The two end points of a frequency polygon always lie on the x-axis.

the basic difference between histogram and frequency polygon is that histogram uses bars to show its frequency and frequeny polygon uses points and join its staight line

In order to plot the points on either the frequency polygon or curve, the mid values of the class intervals of the distribution are calculated. Then the frequencies with respect to the mid points are plotted. However in a frequency curve the points are joined by a smooth curve, where as in a frequency polygon the points are joined by straight lines. Apart from this major difference, a frequency polygon is a closed figure where as the frequency curve is not.

A frequency polygon is not very effective in displaying group data when the class sizes are not the same.