Translate information between graphical and numeric form.

The idea

This skill means moving between a graph and the numbers it represents, in either direction. From a graph, read a value off an axis at a chosen point, or find the rate from the gradient of a straight-line section. From numbers, plot points accurately and join them appropriately. In photosynthesis, a graph of rate against light intensity typically rises then levels off; reading the plateau shows the maximum rate, while the initial gradient shows how quickly rate increases. To go from numeric to graphical, label axes with quantity and unit, choose a sensible scale, plot each pair, then draw a line or curve of best fit. To go from graphical to numeric, identify the required axis value, read across or up to the line, then down or across to the other axis, and state the unit.

What earns the marks

• Read a value from a graph by locating the required point on one axis, moving to the line or curve, then across to the other axis, and quoting the value with its unit.
• Determine a rate from the gradient of a straight-line section by dividing the change in the y-axis quantity by the change in the x-axis quantity, including units.
• Plot supplied numeric data accurately by choosing a scale that uses most of the grid, labelling both axes with quantity and unit, and plotting points to within half a small square.
• Interpret a plateau on a rate graph as the maximum rate under those conditions, and a steeper initial section as a faster increase in rate.
• Translate in reverse by taking values from a table and constructing a line or curve of best fit, recognising when a straight line or a smooth curve is appropriate.
• Use the graph to predict an unmeasured value by interpolation between plotted points, or by extending a straight-line trend where the pattern supports it.

Common mistakes and how to put them right

• Reading the axes the wrong way round, for example treating the x-axis as rate.

  Put it right: Check each axis label and unit before reading any value.

• Joining plotted points with straight segments when the relationship is a smooth curve.

  Put it right: Draw a single smooth curve of best fit unless the data clearly form a straight line.

• Forgetting units or giving the gradient without units.

  Put it right: Divide the y-axis unit by the x-axis unit and state the resulting unit with the answer.

Exam tips

• Annotate the graph with the values you read, showing the construction lines, so your method is visible to the examiner.
• When calculating a gradient, choose two points on the straight section that are far apart and clearly on the line, not two plotted data points.
• Check that your answer's unit matches the axis units, converting if the question uses different units such as minutes and seconds.