Line graphs

A graph made of points joined by straight line segments is a line graph. It is usually used for data over time.

Before reading any graph, ask where the numbers came from. 'Monthly maximum temperature in Kerala' means: weather stations across the state record temperatures, and the month's maximum is the highest value any of them recorded.

Why does that matter? Knowing how the data was collected tells you what it covers, warns you about bias or missing information, and tells you how sure you can be of any conclusion. It shows you the data's limitations.

Then read the graph in two steps.

Step 1: find what is given. How is the graph set out? What scale is used? Which line is which? Here the horizontal axis shows the months, the vertical axis shows temperature in °C, Kerala is blue circles and Punjab is red.

Using different shapes for the markers, not just colours, means the graph still works when printed in black and white, and for readers who cannot easily tell colours apart.

Step 2: work out what it means. In Punjab, the monthly maximum climbs from January to June, peaking at 38°C. It drops to just under 35°C in July, stays about the same until September, then falls steadily to about 23°C in December. January is the lowest, about 19°C.

Kerala is very different: almost flat all year, peaking near 33°C in April and dipping to about 29°C in July. Its summer and winter maximums are nearly the same.

Summed up in one sentence: Punjab's temperature varies much more, getting both colder and hotter than Kerala's. Then let the graph raise questions. Why are these patterns so different? What decides a region's temperature? Which other states look like Punjab, and what do they have in common?

Why a line graph and not columns? Imagine 13 years of data for 4 countries as a column graph: 52 bars, crowded and hard to read. Lines show trends instead. And the steeper a line segment, the faster the change.