What a graph question is marked on
When an exam asks you to plot a graph, the examiner does not mark it on how nice it looks. They tick off a short list. We use SLAPUK to remember that list: Scale, Line, Axes, Plots, Units and Key.
A graph is usually out of 5 marks (SLAPU). When there is more than one line or data set on the same axes it is out of 6 marks (SLAPUK), because the key is needed too.
Build it in this order
Draw a graph in the same order every time, so nothing gets missed.
- Axes (A): find the independent variable (what was changed). It goes on the x-axis. The dependent variable (what was measured) goes on the y-axis.
- Labels and units (A, U): copy the column headings from the table, with the units after a slash or in brackets: "Time / min" or "Temperature (°C)".
- Scale (S): for each axis, divide the range of the data by the number of big squares, then round UP to an easy step: 1, 2 or 5 (or 10, 20, 50 …). Check the data will use more than half the grid. Write the scale on every big line.
- Plots (P): plot every point as a small cross (×) or a circled dot (⊙). Count small squares carefully: each must be within half a small square.
- Line (L): read the question. In biology, unless it asks for a line of best fit, join the points with straight lines using a ruler, from each point to the next. Do not go past the first or last point.
- Key (K): only if there are two or more data sets: say which line is which.
Worked example: Worked example: pondweed and a lamp (task 1 below)
- The distance of the lamp was changed, so distance from lamp / cm goes on the x-axis. The bubbles were counted, so rate of photosynthesis / bubbles/min goes on the y-axis.
- x-axis: the data goes up to 50 cm. 50 ÷ 8 big squares = 6.25 → round UP to 10 cm per big square. The axis runs 0 to 80, and the data uses 50 of 80: more than half. ✓
- y-axis: the data goes up to 48. 48 ÷ 10 big squares = 4.8 → round UP to 5 bubbles/min per big square. The axis runs 0 to 50 and the data uses almost all of it. ✓
- Each small square on the y-axis is worth 5 ÷ 5 = 1 bubble/min, so 36 is one small square above the 35 line. Plot every point with a small ×.
- The question says "Join the points with straight lines using a ruler", so join each point to the next one with the ruler tool. No key: there is only one data set. That is SLAPU: 5 marks.
How a graph is marked: SLAPUK
Out of 5 (S L A P U). Out of 6 (S L A P U K) when there is more than one data set on the graph.
Scale
Linear scale on each axis with even, consistent intervals (1, 2, 5, 10 and so on) and the plotted data covering more than half of the grid in each direction.
Line
Points joined with a ruler. In biology the default is straight ruled lines from point to point (dot to dot), not a line of best fit. Only draw a line of best fit when the question asks for one; then have equal numbers of points above and below it.
Axes
Independent variable on the x-axis, dependent variable on the y-axis, both axes labelled using the headings from the data table.
Plots
Every point plotted accurately, as a small cross or a circled dot, to within half a small square.
Units
Units next to each axis label, after a slash or in brackets, e.g. "Time / minutes" or "Temperature (°C)".
Key
A key that identifies each line or data set. Needed only when more than one line or data set is on the same graph.