A histogram is a graph made directly from a grouped frequency distribution.  Class intervals are represented on the X-axis and the frequency of scores within intervals is denoted by the height of the bar measured against values on the Y-axis.  Measurement on the Y-axis may reflect a simple count value (i.e., 0, 1, 2, 3,...) or may indicate relative frequency (i.e., proportion, 0 to 1.00, or percentage, 0% to 100%).

Emanating from the X-axis are contiguous bars, that is, bars that touch one another.  Click the histogram button on the applet to see the graphic representation of the exam-score data.

Notice how the histogram is constructed from the grouped frequency table.  The height of the bars corresponds to the frequency count for the respective intervals, the width of the bars extends from the exact lower limit to the exact upper limit of the interval, and midpoints are used as labels to represent intervals on the X-axis.