Methods for Testing Normality of Hydrologic Time Series - Hydrologic Time Series Analysis: Theory and Practice - page 33

If necessary, specify an endpoint convention, i.e., what to do with
cases that fall on interval endpoints.
Step 2: Count the number of data points within each data interval. For a
frequency plot with equal interval sizes, the number of data points
represents the height of the boxes on the frequency plot.
Step 3: Decide the horizontal axis based on the data range of time series. The
vertical axis for a frequency plot is the number of data points, while
the vertical axis of the histogram is based on percentages.
Step 4: For a histogram, compute the percentage of data points within each
data interval by dividing the number of data points within each data
interval (Step 3) by the total number of time series data.
Step 5: When the data intervals are not of equal widths for a histogram,
select a common unit that corresponds to the X-axis. Compute the
number of common units in each interval and divide the percentage
of data points within each data interval (Step 4) by this number.
Step 6: Plot the data intervals using boxes, against the results of Step 5 for a
histogram or the intervals against the number of data points in a data
interval (Step 2) for a frequency plot.
Fig. 3.1. Example of a frequency plot.
Fig. 3.2. Example of a histogram.