Be able to select the graph or chart type

Topic 16 – Graphs and Charts

Objective

Be able to select the most appropriate graph or chart type for a given set of data.

Why Choose the Right Graph?

Choosing the correct visual representation helps the audience to:

• Understand trends and patterns quickly.
• Compare values accurately.
• Interpret relationships between variables.
• Make informed decisions based on the data.

Common Graph and Chart Types

• Bar Chart – compares discrete categories.
• Column Chart – similar to a bar chart but vertical; useful for time‑based data.
• Line Graph – shows trends over continuous intervals (e.g., months, years).
• Pie Chart – displays parts of a whole as percentages.
• Scatter Plot – shows relationship between two quantitative variables.
• Histogram – shows frequency distribution of a single quantitative variable.
• Area Chart – like a line graph but fills the area below the line; good for cumulative totals.
• Stacked Bar/Column Chart – compares total size of categories and the contribution of sub‑categories.

Comparison of Graph Types

Graph / Chart Type	Best For	Data Type	Typical Example
Bar Chart	Comparing individual categories	Nominal / ordinal	Sales of different products in a month
Column Chart	Comparing categories over time	Nominal with time series	Quarterly revenue per region
Line Graph	Showing trends or changes	Continuous (time)	Temperature changes over a year
Pie Chart	Displaying parts of a whole	Percentages / fractions	Market share of five companies
Scatter Plot	Examining correlation between two variables	Two quantitative variables	Height vs. weight of students
Histogram	Showing frequency distribution	Continuous quantitative	Distribution of test scores
Area Chart	Cumulative totals over time	Continuous (time)	Total website visitors per month
Stacked Bar/Column Chart	Comparing totals and sub‑components	Nominal with sub‑categories	Sales by product line and region

Steps to Select the Appropriate Graph

1. Identify the purpose of the visualisation (compare, show trend, illustrate proportion, etc.).
2. Determine the type of data you have (categorical, ordinal, continuous, percentages).
3. Consider the number of variables:
   • One variable – bar, column, pie, histogram.
   • Two variables – line, scatter, area.
   • More than two – stacked charts or multiple series.
4. Check the audience’s familiarity with the graph type.
5. Choose a graph that presents the data clearly without distortion.

Example Scenarios

• Scenario A: A school wants to show the percentage of students achieving each grade (A, B, C, D, E).
  Best choice: Pie Chart – clearly displays parts of the whole.
• Scenario B: A company tracks monthly sales over the last three years.
  Best choice: Line Graph – highlights trends and seasonal patterns.
• Scenario C: A researcher compares the number of visitors to five different museums.
  Best choice: Bar Chart – easy side‑by‑side comparison.
• Scenario D: An analyst examines the relationship between advertising spend and revenue.
  Best choice: Scatter Plot – shows correlation and possible outliers.

Key Points to Remember

• Use a pie chart only when the total adds up to 100 % and there are few categories.
• Use a line graph for continuous data over time; avoid it for unrelated categories.
• Bar and column charts are interchangeable; choose orientation based on space and readability.
• Scatter plots require both axes to be quantitative; they are not suitable for categorical data.
• Histograms group data into intervals; they differ from bar charts because the bars touch each other.

Suggested Diagram

Practice Exercise

For each of the following data sets, state which graph type would be most appropriate and justify your choice.

1. Number of books sold by genre (Fiction, Non‑fiction, Mystery, Science, Biography).
2. Average daily temperature recorded over a 12‑month period.
3. Proportion of budget allocated to marketing, research, operations, and administration.
4. Relationship between hours studied and exam score for a group of students.
5. Frequency of test scores grouped in intervals of 10 points (0‑10, 11‑20, …, 91‑100).