Digits instead of crosses
Stacked dot plots group the values into classes, so some detailed information about the values is lost.
A clever way to retain some of this lost detail replaces each cross with a digit (0 to 9) that shows information about the position of the value within its stack.
Nutrition in Africa
You work in an aid agency and want to know which African countries are worst nourished and are therefore most in need of food aid. Information about current nutrition in the countries would be useful.
The stacked dot plot below shows the food energy consumption (in thousand kJoules per capita per week) in each African country for the period 2002-2004.
Drag with the mouse over the crosses to discover the names of the countries.
The crosses only allow you to read off the 'units' digits of the values — Zambia (57.1) and Niger (57.6) are on the same stack of crosses. Select Digits from the pop-up menu. The crosses are replaced by the 'tenths' digits of the values.
The extra information from the tenths digits does not help us to understand the distribution of values in this example, so the dot plot is equally effective here.
At any rate, the energy intake values are unlikely to have been recorded to any more accuracy than their units digits for most countries.
Stem and leaf plots
A stem and leaf plot is basically a stacked dot plot using digits instead of crosses. However the layout of the display is slightly different.
The layout of a stem and leaf plot makes it particularly easy to read off the values that the leaves represent:
The format is most easily understood with an example.
Rain days
The stem and leaf plot below shows the number of rainy days in an African village each year between 1994 and 2013.
Click on any leaf in the plot (a black digit). The corresponding data value is shown above the plot.
Observe that the stem is the 'hundreds' and 'tens' digits of the value and the leaf is its 'units' digit.