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Relative proportions make it easier to process graphic components. For example, graphs are better when they are wider than tall since we read left to right, and titles should be larger than annotation for emphasis and context.
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Data points and measures should take up most of the ink usage to reduce other irrelevant, distracting elements. A high data-ink ratio means that there are relatively few non-data related elements. Strive for higher.
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For a bar graph, there should be one nominal and one ordinal variable, where the bars represent nominal variables and the bar lengths correspond to the ordinal.
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To emphasize the data, the graph must have sufficient contrast in saturation between its data measures (lines, bars, points) and its non-data elements (background).