Here is a bar chart showing the favourite sports of one class. It does its job for one group. But what if we wanted to show our class and the class next door on the very same chart, so we could compare them at a glance?

How could we fit two groups onto one set of axes without it turning into a mess?

Four bar charts appear one at a time. Watch how each one reads its side scale, and how some charts show two bars for every category so we can compare two classes. On the last chart, look for where the two classes differ most.

Today we build a multiple bar chart together, starting from an empty grid. Our data compares two classes on how they travel to school: walk, cycle, car. The chart begins blank, and as the class reads out each number we set that bar's height, colour the two groups differently, and add a key so anyone could read it.

Today we work through four multiple bar charts together, building each one and then reading it carefully before we check.

• First we build favourite fruit for two classes, then read in which fruit the two classes differ most.
• Next we build pets in two classes, and find the category with the biggest difference between them.
• Then two weekend clubs and their activities, again finding the biggest difference between the groups.
• Last, a before-and-after chart: favourite subject before a school project and after it. Because these two bars are the same group at two different times, here we ask the trickiest question of all — which subject's preference changed most.

Today's key points

• A multiple bar chart compares two groups by drawing their bars side by side for each category.
• A key is essential so the reader knows which colour belongs to which group.
• A sensible scale, going up in equal steps, lets every bar be read fairly.
• To find where two groups differ most, look for the category with the biggest gap between its two bars.
• When the two bars are the same group measured before and after, that biggest gap tells us which preference changed most.