← VCE Data Analytics U3O1

Confirming charts and graphs represent the data

Comparing a chart's plotted values, category labels, totals and axis scale with the source data to confirm the chart shows the data accurately.

Part of: VCE Data Analytics - U3O1 - Data Analytics — Module 148, ACARA Digital Technologies Years 9–10 — Analysing and Visualising Data — Module 71

Understanding Confirming charts and graphs represent the data in VCE Data Analytics U3O1

What it does: Calculating the column total from the source gives a figure to compare with the total shown on the chart. Pointing at the source cell shows its real value, which should equal the bar or point plotted for it. Category labels must match the source, with none missing, duplicated or mislabelled. Colour does not affect accuracy. A truncated axis exaggerates differences. When verifying a chart, always check where the axis starts and how it is scaled. A chart may use the wrong cell range. Verify it again against the current source data after any change.

Common mistakes: The chart total must be calculated from the plotted chart values. Using SUM(source) compares the source with itself, so it can never find a difference. It should be totalChart ← SUM(chart). A bar chart axis that starts above 0 exaggerates differences between bars, so the warning must appear when axisMin > 0. Using < 0 would never warn for a truncated axis. It should be IF axisMin > 0 THEN.

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This module's key terms are also in a printable crossword (Modules 130–148) and word search (Modules 94–148).

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Learning
Duo
A column chart shows Maths = 80, but the source table says Maths = 88. What should you do?
Leave it, because the chart looks about rightFix the chart so the plotted value matches the source data (88)
Matches
Column chart of marks with the y-axis starting at 45 instead of 0
↓
The axis does not start at zero, so small differences look huge and the bars do not show the data in proportion. Reset the axis minimum to 0.
Applying
Meanings
Term
Chart verification
Meaning
Comparing a chart's plotted values, category labels, totals and axis scale with the source data to confirm the chart shows the data accurately.
Practice
=IF(SUM(B2:B9)=579,"Total matches","Check total")
Testing
Gaps
=(B2=F2,"Match","Check")
Check Answer
Spots
3totalSrc ← SUM(source)
4totalChart ← SUM(source)

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Quick Questions

1. A column chart shows Maths = 80, but the source table says Maths = 88. What should you do?
A. Fix the chart so the plotted value matches the source data (88) ✓
B. Leave it, because the chart looks about right
Chart verification means each plotted value must match the source data exactly, not just look close.
2. Which check confirms a chart uses the correct categories?
A. Compare each category label on the chart with the labels in the source data ✓
B. Check that the chart uses bright colours
Category labels must match the source, with none missing, duplicated or mislabelled. Colour does not affect accuracy.
3. A column chart's vertical axis starts at 50 instead of 0. What is the risk?
A. The plotted values change in the source data
B. Differences between columns look bigger than they really are ✓
A truncated axis exaggerates differences. When verifying a chart, always check where the axis starts and how it is scaled.
4. A source table has 8 students, but the chart shows only 7 columns. What does this show?
A. The chart does not fully represent the data, so a category is missing ✓
B. The chart is fine, because 7 columns is close enough
Count the categories in the chart and compare with the source. A missing category means the chart is inaccurate.
5. Pie chart slices should add up to which total?
A. Any total, as long as the slices are different sizes
B. 100% of the source data's total ✓
A pie chart shows parts of one whole. Check that the slices add to 100% and that the total matches SUM of the source values.

Match the Pairs

=SUM(B2:B9) → Adds the source values so the total can be compared with the total shown on the chart
=COUNTIF(B2:B9,"Tech") → Counts how many source rows belong to one category so it can be compared with that chart bar or slice
Check that the chart's total matches the total in the source data → =SUM(C2:C9)
Check that a plotted value matches the source cell it came from → =B4
Column chart of marks with the y-axis starting at 45 instead of 0 → The axis does not start at zero, so small differences look huge and the bars do not show the data in proportion. Reset the axis minimum to 0.
Chart series range =Sheet1!$B$2:$B$8 when the data runs to row 9 → The chart range leaves out the last row, so a category is missing and the totals are wrong. Extend the range to B9.

Key Term

Chart verification
Comparing a chart's plotted values, category labels, totals and axis scale with the source data to confirm the chart shows the data accurately.

Fill in the Blanks

1. =IF(B2=F2,"Match","Check")
Hint: Type the function that returns one result if a test is true and another if it is false, used to compare a plotted value with the source.
2. =IF(MAX(B2:B9)<=100,"Axis fits","Rescale")
Hint: Type the function that finds the largest value, so you can check the axis maximum covers all the data.
3. =COUNTA(A2:A9)
Hint: Type the function that counts non-empty cells, to check the number of category labels matches the number of bars.
4. =IF(SUM(B2:B9)=F2,"OK","Wrong total")
Hint: Type the comparison operator that tests whether the source total equals the total shown on the chart.
5. =IF(MIN(B2:B9)>=0,"Axis from 0","Check")
Hint: Type the function that finds the smallest value, to check where the chart axis should start.

Spot the Error

Question 1
A source ← [12, 18, 9, 21]
B chart ← [12, 18, 9, 21]
C totalSrc ← SUM(source)
D totalChart ← SUM(source)
E IF totalSrc = totalChart THEN
F OUTPUT "Totals match"
Line D — The chart total must be calculated from the plotted chart values. Using SUM(source) compares the source with itself, so it can never find a difference. It should be totalChart ← SUM(chart).
Question 2
A barChart ← TRUE
B axisMin ← 20
C IF axisMin < 0 THEN
D OUTPUT "Bars exaggerated"
Line C — A bar chart axis that starts above 0 exaggerates differences between bars, so the warning must appear when axisMin > 0. Using < 0 would never warn for a truncated axis. It should be IF axisMin > 0 THEN.
Question 3
A srcCats ← ["Mon","Tue","Thu"]
B chCats ← ["Mon","Tue","Wed"]
C IF srcCats = chCats THEN
D OUTPUT "Categories match"
Line A — The source categories should be Mon, Tue and Wed. Thu does not match the chart's Wed label, so the check would report a mismatch even though the chart is correct. Line 1 should list "Wed" as the third category.
Question 4
A total ← 100
B shares ← [40, 25, 35]
C FOR EACH s IN shares
D total ← total + s
E IF total ≠ 100 THEN
F OUTPUT "Fix pie"
Line A — A running total must start at 0. Starting at 100 would make the pie slices add up to 200 and give the wrong check. It should be total ← 0.
Question 5
A src ← [30, 45, 60]
B plotted ← [30, 54, 60]
C IF src = plotted THEN
D OUTPUT "Values match"
Line B — The plotted values should be the same as the source values. The 45 has been swapped to 54, which is a typical transposition error. It should be plotted ← [30, 45, 60].

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