Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Constructing bar charts and
histograms
Not all experiments generate results that can be displayed
as a line graph. Some results are best shown in bar charts
or histograms.
A bar chart is drawn when you have a discontinuous
variable on the x-axis and a continuous variable on the
y-axis. A discontinuous variable is one where there is no
continuous relationship between the items listed on the
scale. Each category is discrete. Figure P1.13 shows an
example. The x-axis lists five species of tree; each type of
tree is separate from the others, and there is no continuous
relationship between them. The bars are therefore drawn
with gaps between them.
A continuous variable is one where there is a smooth,
numerical relationship between the values. (Line graphs
always have a continuous variable on both the x-axis
and y-axis, as in Figure P1.12.) Sometimes, you will want
to draw a graph where there is a continuous range of
categories on the x-axis, and the frequency with which
each of these categories occurs is shown on the y-axis. In
this case, the bars are drawn so that they touch. This kind
of graph is a histogram, or a frequency diagram. Figure
P1.14 shows an example.
20
258
Mean number of prickles
18
16
14
12
10
species A
species B
species C
species D
species E
Figure P1.13 Bar chart showing the mean number of prickles
on leaves from five different species of tree.
100
80
Number of leaves
60
40
20
0
8–9
10–11
12–13
14–15
16–17
18–19
20–21
22–23
Number of prickles
Figure P1.14 Frequency diagram (histogram) showing the numbers
of leaves with different numbers of prickles on a holly tree.