Graphs are often referred to as pictures of numbers. Graphs capture the meaning behind a set of numbers and convey that meaning at a glance. As a researcher, you are no doubt familiar with bar charts, which are often used to compare sizes or frequencies of variables: daily calorie intake of people from different countries, for example, or average annual rainfall (millimeters) for different cities.
Since what matters in such charts is the height of each bar (or the length of each bar if the bars are horizontal), it should not matter whether the bars are "empty" (blank) or filled with different colors or patterns (dots, squares, straight lines, etc.). However, such fills are common in charts and serve to add visual interest as well as to make the chart more prominent, and this post is about choosing appropriate fills.
In component bar charts or stacked bar charts, fills are essential to distinguish between different components within a bar. For example, if the average intake of calories is divided into that obtained from proteins, carbohydrates, and fats, or if the annual rainfall is split across seasons, it is necessary to divide each bar into appropriate segments (one each for proteins, carbohydrates, and fats in the first example, and one each to represent a season in the second example above).
If the chart is to be reproduced in color, the choice is wide (although it is better to avoid too many colors and odd combinations of colors). If the chart is to be reproduced only in black and white, it is important to avoid shades or tints of grey (since the differences are usually lost when the chart is photocopied) and patterns that cause eye strain or optical illusions (horizontal and vertical lines for adjacent blocks, for example, or lines at 45° to the horizontal in one direction – north-east to south-west – next to those oriented south-east to north-west).
Lastly, when using bar charts, remember to keep the gap between a pair of bars narrower than the width of each bar. As a rule of thumb, do not make the gap wider than 40% of the width of a bar.
