Stacked area graphs are useful to visualise part-to-whole relationships for extended time series. They are akin to a series of stacked bars over time, where the values for each proportion (i.e. bar) are connected in the same way that time-series data points are connected to form a line graph. The areas between lines are the proportions or parts of the total measure at each time point.
This graph type is appropriate when you want to show that certain parts of a total measure (see graph below), or the relative contribution of those parts to the total measure, are changing over time.
Readers can find it difficult to judge the relative size of stacked parts at individual time points on these graphs. If this is a priority, instead consider a vertical stacked bar graph.
