Plot and draw appropriate graphs selecting appropriate scales for the axes.

The idea

Graph plotting is a core practical skill in biology. When investigating how a factor such as light intensity affects the rate of photosynthesis, you must choose which variable goes on each axis, select a scale that uses most of the grid, and plot points accurately before drawing a line or curve of best fit. The independent variable goes on the x-axis and the dependent variable on the y-axis. Choose a scale so the data spans most of the grid, for example 1 cm per 10 units, and label each axis with quantity and unit. Plot each point with a small cross, then draw a smooth line or curve of best fit rather than joining points dot-to-dot. Examiners assess whether axes, scales, plotting and line of best fit are all appropriate for the data.

What earns the marks

• Label the x-axis with the independent variable and the y-axis with the dependent variable, including correct units such as light intensity in lux or rate in cm³/min.
• Choose a scale that uses at least half of each axis and allows every data value to be plotted, for example 2 cm per 10 °C rather than 1 cm per 100 °C.
• Plot each point accurately using a small cross or dot, reading the scale carefully so points sit within half a small square of the true value.
• Draw a single smooth line or curve of best fit that passes through or near the points, rather than joining points with straight segments.
• Use a ruler for straight-line relationships and a smooth freehand curve for non-linear relationships, extending the line only within the plotted data range unless told otherwise.

Common mistakes and how to put them right

• Putting the dependent variable on the x-axis.

  Put it right: Place the variable you chose to change on the x-axis and the measured result on the y-axis.

• Choosing a scale that leaves most of the grid empty, such as 1 cm per 100 units when data range from 10 to 40.

  Put it right: Rescale so the data fill the grid, for example 1 cm per 5 units.

• Joining points dot-to-dot with straight lines.

  Put it right: Draw a single smooth line or curve of best fit that shows the overall trend.

Exam tips

• Before plotting, check the range of each variable and write the scale on the axis so each major gridline represents a sensible round number.
• Plot points with a sharp pencil and small crosses so the examiner can see exactly where each value lies.
• If the relationship is non-linear, draw one smooth curve rather than a series of straight lines, and do not force the line through the origin unless the data support it.