Relative frequency
When all histogram classes are of equal width, histograms are often drawn with a vertical axis giving the frequencies (counts) for each class. The vertical axis can alternatively be labelled with the relative frequencies (proportions) for the classes.
(There is no harm in including both axes.)
Area equals relative frequency
An important property of histograms is that the proportion of values in one or more classes equals the proportion of the histogram area above these classes.
Therefore,
Relative frequency = proportion of the total area