Line graphs are the best way to depict trend over time, as they are the easiest to understand

Good graph;  quick to understand,
well labelled, communicates a lot of data
The purpose of a graph is to communicate data in a clear, quick and easy to understand way, in order to support and expand upon the text.  It is immediately clear upon looking at this first graph that the percentage of social networking users is increasing, due to the upward trend of the lines.

And not only is it easy to understand, but it is well labelled and communicates a lot of data. From this graph, we know that social networking is increasing, we know the rate of increase due to the gradient and we know when the increases are occurring. We also know that as the age groups get younger, the percentage of users increases.

Poor graph; harder to understand,
poorly labelled and communicates less information
The second graph takes much longer to understand, due to the increases over time being split into age-groups.  Being a bar graph, we can't see the rate or time of increase, thereby communicating less data. We also don't know exact values due to poor labelling.

In this scenario, a line graph was the best graph type to use.

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