Bar Charts, Line Graphs & Pictograms (Edexcel GCSE Statistics)

Bar Charts & Vertical Line Graphs

What is a bar chart?

• A bar chart is a visual way to represent discrete data

  • Discrete data is data that can be counted 

    • This can be numerical like shoe sizes in a class

    • Or non-numerical (categorical) like colours of cars down a road

• The horizontal axis shows the different outcomes

• The vertical axis shows the frequency

• The heights of the bars show the frequency

  • Bars should be separated by gaps

  • Bars should have equal widths

What is a dual (comparative) bar chart?

• Dual (or comparative) bar charts compare two data sets on one bar chart

  • The data sets measure the same variable, so use the same scale

  • The bars are in pairs (side-by-side) for each outcome

  • e.g. For comparing the shoe sizes of two year groups

What is a bar-line chart?

• A bar-line chart also shows two data sets on one chart

  • However one data set is represented by a line, and the other by bars

  • This allows two different variables to be shown, with a different scale for each

  • e.g. For showing the monthly temperature (as a line) and the monthly rainfall (as bars) across the year for a location

    • One scale would be in °C, and the other would be in mm

• There are also composite bar charts, which are covered in their own section below

What is a vertical line graph?

• A vertical line graph (or vertical line chart), is a visual way to represent discrete data
  

  • Vertical line graphs are used for numerical data (rather than categorical data)

    • They are particularly useful when there are lots of different options to show
      e.g.  Results of a test where scores are given as percentages

  • Do not confuse these with the line graphs which are used when drawing time series graphs

    • For those, see the 'Time Series Graphs' revision note

• The vertical axis shows the frequency

• The horizontal axis shows the different outcomes

• You can easily identify the mode (most common value) using a line chart
  

  • This will be the outcome with the highest (tallest/longest) line

  • e.g.  In the line chart above, 11 was the modal test score, with a frequency of 7

• You can quickly see how the data is spread using a line chart

  • Lines may be crowded around a particular group of options

  • This may help identify anomalies or outliers in the data

  • e.g.  In the line chart above you can see

    • the majority of the test scores, out of 20, were between 7 and 12

    • one pupil scored 19 out of 20, much higher than anyone else in the class

Pictograms

What is a pictogram?

• A pictogram is an alternative to a bar chart

  • It is used in the same situations

• There are no axes

  • Frequency is represented by symbols

  • A key shows the value of 1 symbol

    • For example, 1 symbol represents a frequency of 2

  • Half and quarter symbols are often used

• The pictogram above shows the shoe sizes of students in a class

  • As 1 picture of a shoe represents 2 students

    • Half a shoe represents 1 student

  • The number of students with a shoe size of 7, is 3

Composite Bar Charts

What is a composite bar chart?

• A composite bar chart shows the total frequency for a category as well as the proportions in each category

• For example, the chart below shows the total number of vehicles passing a location at different times

  • The overall height of each bar shows the total number of vehicles

  • The sections within each bar show the proportion of vehicles by type

    • Be careful - it is just the height of each coloured section which shows the frequency, not the whole bar

    • e.g. For 9-9:30 am, there are around 10 bikes, not 80

• Composite bar charts can reveal more information that a regular bar chart

  • e.g. The below chart shows the least traffic in total is between 9:30-10 am, but it is also during this time that the greatest number of lorries are present