Stacking the crosses shows density better
Jittering large batches of values can provide an effective display of ranges of high and low densities of values. However the randomness of the jittering can be disconcerting.
A stacked dot plot uses the perpendicular axis more directly to show density. A stacked dot plot is obtained by ...
A computer should always be used to draw stem and leaf plots — you will never need to draw one by hand.
The diagram below illustrates the stacking of crosses in a stem and leaf plot.
Annual rainfall in Samaru, Nigeria
The jittered dot plot below again shows the annual rainfall (mm) in Samaru each year from 1928 to 1983.
Click the button Animate Stacking.
The slider can be used to replay the animation more slowly.
Jittered vs stacked dot plots
For data analysis purposes,
Stacked dot plots show the distribution of values in a data set better than jittered dot plots.
For teaching purposes in CAST however, jittered dot plots are often the most effective way to explain statistical concepts, so it is important that you understand them.
Jittered dot plots are extensively used in CAST.
Grouping and loss of detailed information
Stacked dot plots involve some loss of detailed information about the individual values. The bigger the crosses, the coarser the grouping and the greater the loss of detailed information.
However stacked dot plots more clearly show density through the heights of the stacks.
The loss of detailed information in a stacked dot plot is rarely important.
Select different cross sizes in the diagram above and replay the animation. Large crosses require a coarser grouping of values, so the stacks tend to be higher.
Many of the raw data values were recorded to the nearest cm, so little information is lost from the grouping.