4.5 Visualising numbers
Histograms are used to plot quantitative data, with ranges of data grouped into intervals. For instance, Figure 15 shows a histogram of 200 people using height ranges of 20 mm, with heights measured to the nearest 1 mm. Notice that the values on the horizontal axis are continuous – the first bucket goes from 1600 mm to 1619 mm, the second from 1620 mm to 1639 mm, and so on. The vertical axis shows percentages. The area of each bar represents the percentage of people whose height falls in that range, and the sum of all the areas would equal 100%.
Bar charts can be used to group and count the frequency of anything, using categories that are not necessarily numerical. Figure 16 shows a bar chart representing a packet of sweets – sorting and counting the sweets according to flavour makes it possible to say something useful about how those flavours are distributed.
In Figure 16 the values on the horizontal axis (x-axis) represent a category − flavour in this case − and the vertical axis (y-axis) states how many sweets fell into that category. The bars could have been drawn in any order, but ordering them by size as in this case is often most informative.