Comparing magnitude across separate groups or categories

Data that describe proportions or magnitudes of a measure or measures across different populations, groups, items or categories are most easily understood by readers as a bar chart. Bars are the most familiar shape to readers for understanding differences in data values, and the height or length of side-by-side bars is automatically understood as relative size:

Visualising single part-to-whole relationships

Horizontal bar graphs are effective for readers to judge part-to-whole relationships, particularly when the bars are aligned lengthwise. The inclusion of the horizontal axis in a bar chart further aids comparisons of magnitude.

Pie charts are often used to display proportions of a total measure. The problems associated with this graph type are discussed in Things to avoid. The graph below shows how a traditional pie chart is reconfigured into a more readily understood bar chart (Few 2012):