Lessons · Teaching · reading a graph, and the ones that mislead
Which graph for which question, and the ones that mislead
A bar chart compares separate categories, a line graph shows change over time, a pie chart shows parts of one whole, and a pictograph does what a bar chart does with a key. The choice is decided by the data, not by taste.
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In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.
What it is for
A pie chart of things that do not add to one whole is meaningless, and a line graph across categories that have no order invents a trend that does not exist. Both appear in classrooms constantly, and a teacher who can say why they are wrong can stop them.
How to think about it
Read the axes and the key before the picture. Check where the vertical axis starts, because an axis that does not begin at zero exaggerates every difference on it. Then ask whether the graph type matches the question the data can answer.
Worked example
Categories with no order, like favourite fruit: a bar chart, never a lineA line between apples and bananas implies a journey that does not exist.
Change over time: a line graph, because the horizontal axis is orderedTime gives the order that makes the line mean something.
Parts of one whole that total 100 per cent: a pie chartIf the parts do not make one whole, no pie chart is honest.
An axis starting at 90 rather than 0 makes a 2 per cent gap look enormousThe commonest way an accurate graph tells a lie.
Your turn
Name the graph that shows parts of a single whole.
Parts of one whole are shown by a chart
Solve one, graded on the server
The trap
Reading a pictograph without its key. If one symbol stands for ten, counting symbols gives an answer ten times too small, and the key is always the smallest print on the page.