While line graphs always show changes or trends (increase, decrease etc.), this is not always true for bar charts, pie charts and tables.
Here is a bar chart that does show 'change':
And here is a bar chart that shows 'comparison' rather than change:
Can you see the difference? How will this difference affect what you write in your essay? I'll go into more detail about the two charts next week, but the important thing for now is to see the difference.
(Charts taken from US Department of Transportation website)