Both diagrams, as well as graphs, are ways to represent a given set of data values pictorially. They convert the numerical data into visual form. Their main purpose is to express the data in a compact form so that it becomes readily understandable.

**What are Graphs?**

Graphs are generally used to present the data when both the variables are quantitative. Let us consider an example in economics. Suppose we want to understand the relationship between price and demand. We take a graph paper and plot the numerical values of the demand on the X-axis and price on the Y-axis. We then obtain a line graph by joining the dots. If the line is trending upwards then we can conclude that as the demand for a particular commodity increases so does its price.

**What are Diagrams?**

Diagrams are generally used to represent the data when one of the variables is categorical in nature. Cartograms are a classic example of diagrams in statistics. They are used to depict quantitative facts on a geographical basis. For example, we can show the distribution of rainfall over a particular country by color coding a map. Some other other examples of charts are pie charts and pictograms.

**Differences Between Graphs and Diagrams:**

There are no hard and fast rules for distinguishing a diagram from a graph in statistics. Nevertheless, a few salient points can be kept in mind:

- Graphs are usually drawn on graph paper whereas diagrams are usually drawn on a blank sheet of paper.
- In diagrams, the information is presented using shapes like rectangles, circles (as in pie charts), etc. On the other hand, graphs are constructed using dots, lines, and curves. For example, we plot some points and join them using straight lines when constructing a line graph.
- Diagrams only present information in a general way and skip over the details. For example, a cartogram might give us some idea about the average rainfall in different parts of a country but in order to get the exact values, we would have to look at the original numerical data. On the other hand, graphs are carefully plotted on graph paper with a suitable scale and hence are accurate. We can recover the numerical information of the data from the graph.
- Time series data and frequency distributions cannot be plotted using a diagram. We draw line graphs to represent time series data and histograms to represent continuous frequency distribution.
- The diagrams are generally used when one of the variables is categorical in nature. For example, when plotting the monthly expenditure of a family on a pie chart, some of the categories might be food, rent, clothing, etc. On the other hand, graphs are not generally used to represent categorical data.
- Graphs are much more easier to construct compared to diagrams.