Bars should include all parts of the total measure so that readers are fully informed about which parts contribute most and least. Accordingly, the parts or segments of each bar should add to 100% if data are presented as percentages, or the total absolute value of the measure (eg total greenhouse gas emissions).
There are 2 exceptions to using stacked bar graphs for part-to-whole relationships:
The direction of stacked bar graphs can be differentiated in the same way as for regular bar graphs – horizontal bars are best used for comparing discrete categories or groups (eg categories of primary industry), and vertical bars are best for comparing data values across a relatively small number of time points (about 8 or less).
Caution! Be aware that readers will find it difficult to compare sections across bars when the sections are small. This occurs when sections are competing for space (ie bars include more than about 5 sections) or when the graph itself is small. Consider instead using multiple graphs or a trellis graph under these circumstances.
