Drawing Graphs

What this skill is for

Drawing graphs is a core science process skill assessed in Paper 3 and in the data-response questions of Paper 2. You are given a table of results and asked to display them as a graph, then to read values, describe trends, or calculate a rate.

Marks are awarded for the axes, the scale, accurate plotting, and a correct line of best fit, so the skill is worth practising until it is automatic.

In biology the manipulated variable goes on the horizontal x-axis and the responding variable on the vertical y-axis, so reading a graph correctly also depends on identifying these two variables first.

Apparatus for graph work

• Graph paper with 1 mm squares
• A sharp pencil
• A transparent ruler
• An eraser
• The completed results table

Choosing the right graph type

• Line graph, used when the manipulated variable is continuous (numerical), such as time, temperature, or concentration, allowing values between plotted points to be estimated.
• Bar chart, used when the manipulated variable is discrete or categorical, such as type of leaf, blood group, or species, where values between categories have no meaning.

Setting up the axes

• The manipulated variable is always plotted on the horizontal x-axis, and the responding variable on the vertical y-axis.
• Each axis must be labelled with the name of the variable and its unit in brackets, such as 'Time (min)'.
• Choose a scale that uses at least half of the available graph paper in both directions, with even, easy-to-read intervals.
• Where appropriate, the scale should start from zero, or a break in the axis should be shown clearly if it does not.

Plotting and drawing the line

1. Plot each data point accurately using a small, neat cross or dot at the correct coordinates.
2. For a line graph, draw a single smooth curve or straight line of best fit that passes through or close to as many points as possible, do not join the points dot-to-dot.
3. Give the graph a suitable title stating what is being shown.
4. To read a value from the graph, draw a faint dotted line from the axis to the curve, and across to the other axis, at the point of interest.

Worked example with a data table

Suppose an investigation measures the volume of oxygen given off by pondweed at different light intensities. The results are shown below.

To graph them, put light intensity on the x-axis and volume of oxygen on the y-axis, choose a scale that fills the paper, plot each point, and draw a smooth curve of best fit.

Reading the graph, the volume rises steeply at first and then levels off, which tells you that at high light intensity another factor, such as carbon dioxide concentration or temperature, has become the limiting factor. To find the rate at a chosen intensity, read the volume there and divide by the time.

Reading values and calculating a rate

• Interpolation: reading a value that lies between plotted points, by drawing dotted lines from the axes to the line of best fit.
• Extrapolation: extending the line beyond the plotted range to estimate a value, which should be done with caution because the trend may not continue.
• Rate: the change in the responding variable divided by the time taken, read from the graph as the gradient of a straight portion.
• Gradient: choose two points far apart on the straight part of the line, then divide the change in y by the change in x, giving the rate with its correct unit.

Common mistakes

Paper 3-style questions

Question 1 (choosing and setting up). A student measures the number of air bubbles released by pondweed each minute at five temperatures. State the type of graph that should be drawn and how the axes should be labelled.

Model answer. A line graph should be drawn because temperature is a continuous variable. The x-axis is labelled 'Temperature (°C)', the manipulated variable, and the y-axis is labelled 'Number of bubbles per minute', the responding variable, each with a suitable even scale.

Question 2 (plotting rules). State three things an examiner looks for when marking a plotted graph.

Model answer. The examiner looks for a scale that fills at least half the grid with even intervals, points plotted accurately with small crosses or dots, and a single smooth line or curve of best fit rather than dot-to-dot joining; the axes must be labelled with units and the graph given a title.

Question 3 (reading and inference). From a graph the bubble count rises from 10 °C to 35 °C and then falls sharply. State what the graph shows and give the inference.

Model answer. The graph shows the rate increases with temperature up to about 35 °C, the optimum, and then falls. The inference is that the process is enzyme-controlled: a higher temperature raises the rate until the enzymes begin to denature above the optimum, so the rate drops.

Source:SRC-DSKP-EN

Frequently asked questions