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A scaled bar graph uses bars to represent data with a scale on the axis (0, 5, 10, 15, ...). Read the top of each bar against the scale to find the value.
Create bar graphs with class data using graph paper. Interpret graphs with different scales.
Misreading scale values; misaligning bars with grid lines; forgetting labels.
You already know how to read a basic bar graph where each square on the axis equals 1. A scaled bar graph works exactly the same way — except each square on the axis represents more than 1. Instead of counting squares, you read the number the bar reaches and that number is the value directly. The scale is just a more efficient way to display large data without drawing hundreds of squares.
The most important thing to do when you first look at any graph is check the scale. Look at the y-axis (the vertical axis with numbers) and ask: what does each interval represent? If the axis goes 0, 5, 10, 15, 20 with one grid line between each label, then each line represents 5 units. If a bar ends right on the 15 line, the value is 15. If a bar ends halfway between 10 and 15, the value is about 12 or 13 — you have to estimate when bars land between lines.
This is where most errors happen: students read the grid line number as if the scale were 1, when it might be 2, 5, or 10. For example, if the scale increases by 5 and a bar reaches the line labeled "4," that label means 20 (because each mark goes up by 5: 5, 10, 15, 20). But if you misread it as counting by 1s, you'd say the value is 4. Always identify the scale first, before reading any bars.
When you create a bar graph, you also have to choose a scale. The scale should make the graph fit on the page without making bars impossibly tiny or running off the top. If your largest value is 40, a scale of 1 would require 40 grid lines — too many. A scale of 5 gives you just 8 lines, which is much cleaner. Choosing a good scale is a judgment call, but the rule is simple: pick the smallest interval that keeps all the bars visible and reasonably sized.