Bar graphs and line graphs are fundamentally different in how they visually represent data. Depending on the type of data, one or the other graph may better show trends and comparisons between groups.
Bar Graphs

Bar graphs give readers a quick picture of trends and comparisons between groups. Blocks representing a particular data group are shown against a scale. The height of the block indicates the value of the data group on the scale. Use a bar graph when you want to compare values between more than one group  such as number of sales per employee  or to analyze changes over time  such as the number of sales over four quarters per employee. Bar graphs work best when representing large changes over time.
Line Graphs

Line graphs show changes using a line that connects data points at certain moments in time, such as tracking the average grade in math in a particular classroom over a school year. A line graph is also a good choice if you have data for more than one group to show comparisons over a certain time period. In this case, each data group would have its own line on the graph. A line graph can display high and low points, quick or slow changes, or a trend toward stabilization. In addition, line graphs can have more than two scales to compare a single value over more than one time period. Line graphs are better suited to displaying smaller changes than bar graphs.
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