Line graphs and polygraphs

Line graphs show time series: data that change over time, such as temperature, rainfall, population growth, or birth and death rates.

To draw one:

• round the data to convenient figures;
• draw two axes, with time on the X-axis (horizontal) and the values on the Y-axis;
• choose a scale for the Y-axis, including negative values if needed (India's population growth, for example, was negative in 1911-21);
• plot the points and join them with a line.

A polygraph is a line graph with two or more variables on the same axes, shown by different line styles (solid, dashed, dotted) or colours.

Example: the sex ratio of three states over the decades can be shown as three lines on one polygraph, so the trends can be compared at a glance.