Line graphs

The idea

A line graph marks data points and connects them with line segments; it is generally used to show change over time and shows trends more clearly than many columns would.

Twelve months of temperature

Here is the monthly maximum temperature of Punjab, in °C (rounded).

The values are rounded and match what the book describes for its graph.

What is the easiest way to see how the temperature changes from month to month?

• Month | °C | Month | °C
• Jan | 19 | Jul | 34
• Feb | 23 | Aug | 34
• Mar | 28 | Sep | 34
• Apr | 35 | Oct | 32
• May | 37 | Nov | 28
• Jun | 38 | Dec | 23
• Read the twelve numbers one by one
• Mark a dot for each month and join the dots
• Draw twelve separate columns

Build a line graph

The columns show Punjab's temperature. Mark the top of each column, then join the dots. After that, add Kerala.

Data points joined by line segments

A line graph marks data points and connects them with line segments; it is generally used to show change over time and shows trends more clearly than many columns would.

Each dot is a data point: one month and its temperature. Joining the points with line segments makes the line graph. The horizontal line shows the months and the vertical line shows the temperature in °C.

Read a line graph in two steps. Step 1: what is given? Look at the axes, the scale and the markers. Squares and circles tell the two states apart even in black and white. Step 2: what does it tell us? Punjab rises from January to June and falls after September. Kerala stays almost flat.

Two states over 12 months need 24 columns. Two lines show the same trends at a glance.

Notes

A line graph marks data points and connects them with line segments; it is generally used to show change over time and shows trends more clearly than many columns would.

Check yourself

In which month is Punjab's maximum temperature the highest?

What does the graph show about Kerala and Punjab?

The rainfall data has 6 cities and 12 months. A column graph with one column for each city in each month needs how many columns?

Answer: 72

6 cities × 12 months = 72 columns. A line graph needs only 6 lines.

Which data is best shown on a line graph?

• April. In April it is 35 °C and still rising. The line keeps climbing until June.
• May. In May it is 37 °C. The line goes higher still in June, to 38 °C.
• June — correct. Yes! The red line peaks in June, at 38 °C.
• July. In July the line dips to 34 °C.
• Kerala's temperature rises until June, like Punjab's. Kerala peaks in April, at 33 °C, and is lower in June, 30 °C.
• Kerala's line is almost flat: its maximum changes very little — correct. Yes! Kerala stays between 29 °C and 33 °C all year. Punjab goes from 19 °C to 38 °C.
• Kerala is hotter than Punjab in every month. From April to August Punjab is hotter, 34 to 38 °C against 29 to 33 °C.
• Both lines go up and down in the same way. Punjab swings by 19 °C during the year. Kerala swings by only 4 °C.
• The height of a plant, measured every week — correct. Yes! Time runs along the bottom, so the line shows how the height changes from week to week.
• The favourite fruit of 30 children. These are separate groups, not change over time. A bar graph or a pictograph fits better.
• The number of students in each of four houses. These are four separate groups. Nothing changes over time, so a bar graph fits better.
• The colours of the shirts students wear today. These are separate groups too. A line graph is for data across time.