Plant organ system: plot and draw appropriate graphs, selecting appropriate scales for axes

The idea

Graph plotting is a core practical skill tested across biology, chemistry and physics. You must choose a sensible scale for each axis, label both axes with quantity and unit, and plot points accurately before drawing a line or curve of best fit. For example, in a plant transport investigation measuring water uptake at different light intensities, put light intensity in lux on the x-axis and water uptake in cm³ on the y-axis. Choose a scale so the data fills most of the grid, using equal steps such as 1, 2, 5 or 10 units per major square. Plot each point with a small cross, then draw a smooth line or curve that follows the trend rather than joining every point dot-to-dot. Assessed through practical questions and required practicals where you interpret and present data.

What earns the marks

• Axes are labelled with the correct quantity and unit, for example 'water uptake in cm³' and 'light intensity in lux'.
• A consistent scale is chosen for each axis, using equal intervals such as 1, 2, 5 or 10 units per major grid square.
• The scale uses most of the available grid so the plotted data occupies a large area and is easy to read.
• Points are plotted accurately with small crosses or dots at the correct coordinates.
• A line or curve of best fit is drawn smoothly through or near the points to show the overall trend.
• The independent variable is placed on the x-axis and the dependent variable on the y-axis.

Common mistakes and how to put them right

• Joining every point dot-to-dot instead of drawing a smooth line or curve of best fit.

  Put it right: Draw one smooth line or curve that shows the overall trend.

• Choosing an awkward scale such as 3 or 7 units per square, which makes plotting slow and inaccurate.

  Put it right: Use simple multiples such as 1, 2, 5 or 10.

• Forgetting units on axis labels or writing only the quantity.

  Put it right: Write both the quantity and its unit, for example 'time in s'.

Exam tips

• Check that your scale allows every data point to fit on the grid before you start plotting.
• Use a sharp pencil and a ruler for straight lines, and keep crosses small so the exact point is clear.
• If the trend is a curve, draw it freehand in one smooth movement rather than as a series of short straight sections.