Plotting Graphs

Why this skill matters

Graph plotting is tested on every Paper 6 and frequently carries 4-5 marks. The marks are awarded for specific features: labelled axes, sensible scales, accurate plotting, and an appropriate line of best fit. Each feature has a mark, so missing any one of them costs you directly.

Method and technique

Axes

1. The independent variable (what you change) goes on the x-axis (horizontal).
2. The dependent variable (what you measure) goes on the y-axis (vertical).
3. Label each axis with the quantity name and unit, matching the table headings (e.g., “Time / s” on the x-axis, “Volume of gas / cm3” on the y-axis).

Scale

1. Look at the range of your data. The scale must allow all points to be plotted and the plotted points must occupy at least half the grid area.
2. Choose convenient intervals: 1, 2, 5, 10, 20, 50, etc. Avoid scales that make plotting difficult (3s, 7s, 9s).
3. The scale does not need to start at zero. If your data ranges from 20 to 80 degrees C, starting at 0 wastes grid space.
4. Mark the scale evenly: every major gridline must be labelled.

Plotting points

1. Plot each point accurately using a small, neat cross (x) or a dot with a circle around it. Large blobs hide the precise position.
2. Check each point by reading its coordinates back from the graph. If they do not match the table, replot.

Line of best fit

1. If the trend is linear: use a ruler to draw a single straight line that passes as close to as many points as possible, with roughly equal numbers of points above and below the line. The line does not need to pass through every point or through the origin unless the data demands it.
2. If the trend is curved: draw a smooth, freehand curve that follows the general pattern of the points. Do not use a ruler for curves.
3. Never join the dots point to point with straight segments (unless specifically told to do so). This is not a line of best fit.

Anomalous points

If one point is clearly far from the pattern of the others, it is anomalous. Circle it and do not include it in the line of best fit. Note it in your analysis as a possible source of error.

Common errors and how to avoid them

Scale uses more than half the grid for the axis labels and less than half for data: This means the plotted area is too small. Adjust the scale.

Axes not labelled or units missing: Each axis must have a label with the quantity and unit. No label means losing a mark.

Joining the dots: This produces a jagged line that does not represent the underlying trend. Always draw a best-fit line or curve.

Plotting errors: Misreading the scale or plotting the wrong value. Double-check each point against the table.

Forced through the origin: Only draw through (0,0) if the data supports it and the question expects it. Forcing a line through the origin when the data does not pass through it distorts the graph.

Marking points examiners look for

• Axes correctly labelled with quantity and unit
• Sensible scale (plotted points fill at least half the grid)
• All points plotted accurately (within half a small square)
• Appropriate line of best fit (smooth line or curve, not dot-to-dot)
• Anomalous points identified (circled, excluded from the line)