Be able to adjust the maximum and minimum values of an axis scale and set incremental values

Topic 16 – Graphs and Charts

Objective

Be able to create, edit and label a chart; adjust the minimum and maximum values of an axis scale; set major and minor units; add a second data series (including a secondary axis); and format numbers (currency, decimal places, percentages) in accordance with the Cambridge IGCSE ICT 0417 syllabus.

1. Choosing the appropriate chart type (AO3)

Select the chart type that best represents the data and the information required in the question.

2. Chart‑components checklist (AO1 – Knowledge)

When you look at any chart, verify that the following elements are present and correctly labelled. Use the exact syllabus terminology.

3. Why adjust axis scales? (AO3 – Analyse/Evaluate)

• Ensures the chart starts at a logical point (often zero for quantities).
• Prevents excessive empty space above the highest data point.
• Provides a consistent, easy‑to‑read grid that highlights trends.
• Helps avoid misleading visual impressions (e.g., exaggerating small differences by using a very narrow range).

Exam‑style prompt

Explain why a 0 – 1000 scale with a major unit of 200 is appropriate for the sales data in the example below.

Model answer (2 sentences)

The minimum of 0 £ gives a meaningful baseline because sales cannot be negative, and the maximum of 1000 £ is the next “nice” round number above the highest value (890 £), ensuring all columns are fully visible. A major unit of 200 £ produces five evenly spaced grid lines (0, 200, 400, 600, 800, 1000), which makes the chart easy to read without overcrowding.

4. Key terms (AO1)

• Minimum value – lowest number shown on an axis.
• Maximum value – highest number shown on an axis.
• Major unit – step size between successive major tick marks / grid lines.
• Minor unit – optional smaller step between major units.
• Secondary axis – an additional Y‑axis (or X‑axis) used for a second data series with a different scale.

5. General procedure for setting axis scale (AO2 – Practical Skills)

1. Select the chart to activate the Chart Tools (or Format Chart Area) pane.
2. Click the axis you wish to modify – usually the vertical Y‑axis.
3. Open the Axis Options dialog (right‑click → “Format Axis” or use the pane).
4. Enter the required values:

   • Minimum – e.g., 0 or the lowest data point rounded down.
   • Maximum – a round number ≥ highest data point.
   • Major unit – the increment between major grid lines.
   • Minor unit (optional) – usually Major unit ÷ 2 or ÷ 5.

5. Click OK (or “Close”) and review the chart.

6. Adding a second data series and a secondary axis (AO2)

1. With the chart selected, go to Chart Tools → Design → Select Data.
2. Click Add and specify the new series name and cell range.
3. Right‑click the new series on the chart and choose Format Data Series.
4. In the pane, select Series Options → Plot Series on → Secondary Axis.
5. The chart now shows two Y‑axes (primary on the left, secondary on the right). Adjust each axis scale using the steps in Section 5.
6. Update the legend so each series is clearly identified.

Decision tree – When is a secondary axis necessary?

• Do the two data series use different units (e.g., £ and %)? – Yes → use secondary axis.
• Is the range of one series much larger than the other (e.g., 0‑1000 vs 0‑5)? – Yes → use secondary axis.
• Can both series be displayed clearly on a single axis without distortion? – No → use secondary axis.
• Otherwise, keep a single axis.

7. Formatting numbers (currency, decimal places, percentages) (AO2)

1. Select the data labels, axis numbers or cells you wish to format.
2. Right‑click and choose Format Cells (or use the toolbar’s Number Format dropdown).
3. In the Format Cells dialog, pick the appropriate category:

   • Currency – e.g., £1,230.00 (set symbol and decimal places).
   • Number – control decimal places (e.g., 12.5 vs 12.50).
   • Percentage – display as 45 % (Excel multiplies by 100 automatically).

4. Click OK.

Where to find the dropdown: In Excel/Calc the “Number Format” dropdown is on the Home tab, next to the font‑size box. It shows icons for General, Number, Currency, Accounting, Short Date, Long Date, Percentage, etc.

Ensure the same formatting is applied consistently to axis labels, data labels and any legend entries.

8. Example scenarios

8.1 Column chart – sales data (currency)

Scaling decisions

• Maximum data point = £890 → round up to 1000.
• Minimum = 0 (zero is a logical baseline for sales).
• Range = 1000 − 0 = 1000.
• Desired major grid lines: 5–6 → Range ÷ 5 = 200 (a “nice” number).
• Minor unit = 200 ÷ 2 = 100 (optional).

Resulting axis values: 0, 200, 400, 600, 800, 1000.

8.2 Pie chart – market‑share data (percentages)

Key points for a pie chart:

• Only one data series is required.
• Display data labels (percentages) directly on each slice for clarity.
• Do not use a secondary axis; the whole circle already represents 100 %.

8.3 Scatter plot – height vs. weight (no axis scaling needed for units)

Typical scaling choices:

• Y‑axis (Weight) – Minimum = 40 kg, Maximum = 70 kg, Major unit = 10 kg.
• X‑axis (Height) – Minimum = 145 cm, Maximum = 175 cm, Major unit = 10 cm.
• Both axes use the same units throughout the plot; no secondary axis is required.

9. Tips for choosing appropriate values (AO3)

• Include zero as the minimum unless the context specifically excludes it (e.g., temperature above freezing).
• Round the maximum up to a “nice” number (multiple of 10, 20, 50, 100).
• Pick a major unit that yields 5–10 major grid lines; round the calculated unit to a convenient figure.
• If the data range is very small, consider a narrower minimum‑maximum pair to accentuate differences, but keep the scale logical.
• Ensure the major unit divides evenly into the range; otherwise the top grid line will fall short of the maximum.

10. Common pitfalls

• Setting a minimum above zero when zero is a meaningful baseline.
• Choosing an increment that does not divide evenly into the range, leaving a gap at the top.
• Relying on automatic scaling without checking whether it hides trends or exaggerates them.
• Forgetting to update the legend after adding a second data series.
• Inconsistent number formatting (e.g., mixing currency with plain numbers).
• Omitting units from axis titles or data labels.

11. Summary table

12. Practice exercise – temperature line graph (AO3)

Using the data below, decide on appropriate minimum, maximum and increment values for a line graph. Then justify your choices.

Answer format (example)

Minimum = 0°C
Maximum = 20°C
Increment = 5°C
Justification: 0°C provides a clear baseline; the highest temperature is 15°C, so 20°C (a round number) ensures all points are visible. A 5°C major unit gives four grid lines (0, 5, 10, 15, 20), which is easy to read.

13. Exam checklist (AO2 & AO3)

1. Identify the highest and lowest data values.
2. Decide whether zero should be included as the minimum.
3. Round the maximum up to a convenient number.
4. Calculate a suitable major unit (range ÷ 5‑8, then round).
5. Enter Minimum, Maximum, Major unit (and Minor unit if required) in the Axis Options.
6. If a second data series is needed, add it and decide whether a secondary axis is appropriate (use the decision tree).
7. Format all numbers consistently (currency, decimal places, percentages).
8. Check that the chart title, axis titles (with units), legend, data labels (if required) and gridlines are present and correctly labelled.
9. Review the chart to ensure:

   • All data points are visible.
   • The grid is easy to read.
   • The scaling does not mislead the viewer.

10. Write a brief justification for the chosen chart type and scaling decisions (AO3).