How can I compare two graphs when their scales are different?

Start with the axes, not the apparent shape of the lines or bars. For each graph, write down the vertical-axis minimum and maximum, the gap between labeled ticks, and the unit being measured. Also check the horizontal axis: are the same dates, groups, or other categories included, in the same order?

For example, a vertical axis running from 0 to 100 in steps of 20 gives a different visual frame from one running from 40 to 60 in steps of 5. Changing axis scaling can substantially change how a chart appears and can overemphasize or deemphasize a trend [S1]. Matching-looking slopes or bar heights alone do not show that two sets of values changed by the same amount.

Read the plotted values and compare the actual difference, not just the amount of space the change occupies on the page. If the values are comparable, express both changes in the same units. For example, suppose Graph A goes from 50 to 60 units and Graph B goes from 48 to 58 units. Both rise by 10 units, even if one line looks steeper because its vertical axis covers a narrower range.

If the starting values differ, decide whether you need to compare absolute change or relative change. In this example, both absolute changes are 10 units, but the starting points differ. You could calculate each percentage change as the change divided by the starting value, then multiplied by 100, if relative change is what your question asks about. State which comparison you chose: the graph alone does not determine what counts as meaningful.

First check whether the units can be expressed in a common form. If so, convert the values before comparing them and label the converted unit clearly. If they measure different kinds of things, do not treat the raw numbers as interchangeable; explain the difference or compare a shared measure only if one is provided.

Next, match the categories or time points. If one graph shows January through June and the other shows January through April, compare only the months both include, or note that the coverage differs. Watch for categories that are missing, reordered, grouped differently, or labeled in a way that changes what each value represents.

Use this sequence and jot down the answers before describing the pattern:

1. Range: Record each axis’s minimum and maximum. Does either axis start above zero or cover a much narrower span?
2. Intervals: Note the value represented by each tick gap. Are the steps equal, and are they the same on both graphs?
3. Units: Identify what each number measures. Convert only when the units describe comparable quantities.
4. Coverage: Match the dates or categories, and note anything missing, reordered, or grouped differently.
5. Values and change: Compare corresponding values, then calculate the difference. If relevant, compare percentage changes separately.
6. Conclusion: Describe what the numbers show and mention any scale or coverage difference that could affect the visual impression.

Copyable note: “Graph A uses [range] in [intervals] [unit]; Graph B uses [range] in [intervals] [unit]. Both show [shared categories]. From [start] to [end], A changes by [amount] and B by [amount]. The visual impression [does/does not] match the numerical comparison because [reason].”