11.6 - Bar Charts & Pie Charts
Understanding and reading bar charts
Bar charts are a straightforward way to display and compare data using rectangular bars. They are commonly used to show quantities or frequencies across different categories.
Reading bar charts effectively
- Locate the category - Identify the specific bar on the horizontal axis that corresponds to the data you're interested in.
- Read the value - From the top of the bar, trace horizontally to the vertical axis to determine the value. Pay attention to the units on the scale.
Example: In a bar chart showing annual wheat production for three countries, if the bar for "Grainland" reaches up to 540 on a scale labelled "Thousand tonnes per year", the actual production is 540,000 tonnes.
Constructing bar charts accurately
Creating a bar chart requires precision to ensure the data is represented clearly and correctly.
Drawing bar charts neatly
- Find the value on the scale - Locate the number you need to represent on the vertical axis and use a ruler to draw a faint horizontal line across.
- Draw the bar - Using a ruler, construct a bar up to this line, ensuring it matches the width of other bars in the chart.
Example: To show that "Cropville" produces 320 thousand tonnes of wheat per year, find 320 on the vertical scale, draw a horizontal line across to the "Cropville" category, and then draw a bar up to this line with the same width as the others.
Interpreting pie charts with and without scales
Pie charts are useful for showing proportions or percentages of a whole.
Reading pie charts accurately
- Using a scaled pie chart - If the chart has a percentage scale around its edge, note the starting and ending points of each segment. Subtract the start from the end to calculate the percentage for that category. For instance, if a segment labelled "Train" starts at 10% and ends at 40%, it represents 40 - 10 = 30% of the total.
- Estimating without a scale - For pie charts without markings, visually compare the segments to familiar fractions. A segment that looks like half the circle is approximately 50%, while one that appears as a quarter is about 25%. Use any nearby scaled pie charts as a reference to improve accuracy.
Example: In a pie chart without a scale showing transport preferences, if the "Bus" segment appears to take up roughly a third of the circle, estimate it as approximately 33%.
Drawing pie charts to represent percentages
Constructing a pie chart involves dividing a circle into segments that correspond to specific percentages.
Constructing pie charts correctly
- Mark the starting point - Begin at 0% on the circle's edge and use a ruler to draw a line from the centre to this point.
- Calculate and mark the end point - Determine the percentage for the segment, add it to the starting point, and draw a line from the centre to this new percentage mark on the edge. For example, to draw a segment for 30%, start at 0% and end at 30%. For a second segment of 25%, start at 30% and end at 55% (30 + 25).
- Complete the segment - The area between the two lines represents the percentage for that category.
Example: To create a pie chart showing expenditure, start by drawing a segment for "Housing" at 35% (from 0% to 35%). Then, for "Food" at 18%, draw the next segment from 35% to 53% (35 + 18). Continue this process for remaining categories.