Displaying prior knowledge about grouping of individuals
Occasionally the values in a dot plot or stem and leaf plot separate into clusters, but this is rare. However we sometimes know beforehand that the individuals belong to two or more groups.
Dot plots or stem and leaf plots should be modified to show this extra information. Different colours or symbols might be used to distinguish the groups. However it is easier to compare the groups if they are separately displayed against a common axis.
Rice yields
The display below shows the rice yields (tonnes/hectare) in the hundred countries with highest rice production in 2012.
(Note that Central America has been grouped with South America, North Asia has been grouped with West Asia, and Australia has been included in East Asia.)
Click on crosses to display the names of the countries.
To look for regional differences, we can group the countries into regions and use colour to distinguish them. Click the checkbox Colour groups to do this.
Finally click the button Animate Grouping to separate the groups. Regional differences are clearest in this display.
Some differences between the regions stand out. In particular,
(The demonstration can be repeated with jittered dot plots by choosing jittered instead of stacked from the pop-up menu.)
Back-to-back stem and leaf plots (optional)
For effective comparisons, all dot plots must be drawn against the same axis. Using this principle is harder for stem and leaf plots, but is possible when there are only two groups, using a central column of stems. The leaves for one group are drawn to the right of the stems, and those for the other group are drawn to the left, giving a back-to-back stem and leaf plot.
Rice yields
As an illustration, a back-to-back stem and leaf plot comparing African rice yields to those in Central and South America is shown below.
Click on leaves to display the countries.