Extra information
When only a single value is known from each individual (or plant, item, etc), all that can be revealed is the shape of the distribution of these values. However there is often additional information available which can be used in conjunction with dot plots or stem and leaf plots to give more insight into the data.
In some data sets, each individual or item has a unique name — a textual label. Even this extra information can provide insight into the data in a dot plot or stem and leaf plot.
Wheat yields
The following stacked dot plot shows the wheat yields (tonnes per hectare) of the countries producing over 1 million tonnes of wheat in late 1996 or early 1997.
Drag the mouse over the crosses to see which countries (and regions of the world) each cross refers to. Does this tell you anything more about the data?
Heights of states in USA
In the next example, knowledge of the names of the items from which the values were measured again helps us to understand the variation in the data.
The stacked dot plot shows the heights (ft) of the highest points in each of the states in the USA. As with the wheat yields example, drag over the crosses to identify the states. What extra information can you extract from the state names?
You should observe that the outlier is Alaska, which is also an outlier geographically! Also, the cluster of high values corresponds mostly to states in the west of the USA which contain parts of the Rockies.
The next page describes a different type of extra information that may be available about each individual.