Bar Charts and Pictograms

Picturing frequencies

A frequency table is a list of numbers — a chart turns those numbers into a picture you can read at a glance.

A bar chart draws each category as a bar. Every bar is the same width with a gap between it and the next, and its height — read off the labelled axis — gives that category's frequency. Taller bar, bigger count.

A pictogram instead uses a symbol to stand for a number of items. A key tells you how many each symbol is worth — for example \text{🙂} = 5 — so three symbols mean 3 \times 5 = 15. A part-symbol (half a face, say) shows a part of that number, so half a symbol here would mean 2.5.

Reading the picture

Both charts show the same thing — the frequencies — in two different visual languages. The trick is the same each time: find out what one unit of the picture is worth, then read it off carefully.

On a bar chart, each bar's height = the frequency, read from the labelled axis.
Bars all have equal width with gaps between them — that's what separates a bar chart from a histogram.
On a pictogram, the key says how many each symbol represents; part-symbols show parts of that number.
Always read the axis or key carefully before counting.

Building a bar chart

Step through it: first the frequency axis with its ticks, then a bar for each of four categories — heights 3, 5, 8 and 4.