Churchill Archive for Schools - Guide to Primary Sources_Charts and Gr

Interpreting Charts and Graphs

Why are charts and graphs useful primary sources?

Charts and graphs created by military or other government sources can tell historians what information officials had at the time when they were making critical decisions. This chart from 1940 in the example below shows U-boat activity in conjunction with the number of merchant ships sunk and the number of U-boats destroyed.

How to interpret charts


Source: CHAR 20/135/19

Description: Map charting the supply routes to Britain.

Date: 1940


1. Think about what this source is

This source shows the number of U-boats (German submarines) operating, the number of U-boats sunk, and the number of merchant ships sunk from July to December 1940.

2. Consider the background.

German submarines were an important component of the Axis strategy. By destroying merchant ships and posing a threat to military vessels, U-boats disrupted supply lines and made naval missions in the Atlantic risky. Supplies of food and essential war materials from the rest of the world were essential in order to keep Britain’s war effort alive.

3. Consider what surface level information you can interpret from the chart

Ask yourself:

  • When were the peaks in U-boat activity?
  • When were the most merchant ships destroyed?
  • When were the most U-boats destroyed?

4. Consider what deeper level information you can interpret from the chart

Ask yourself:

  • Do you notice any correlations between peak numbers of U-boats in operation and peak destruction of merchant vessels?
  • After U-boats were destroyed, was there a drop in merchant vessel sinking?
  • How do you think this chart might have been used by military officials? How might historians use it to understand the rationale behind their decisions?

Need more help?

For more information about U-boats, explore our investigation on the Battle of the Atlantic.

How do I interpret other types of primary source?

Choose from the types of primary source below to explore detailed examples that show useful ways to interpret each category, with handy tips and questions to ask yourself: