A bar graph is used for things that only come in predefined quantities, or to compare similar things. You might have a bar chart to compare the numbers of hedghogs, foxes, and badgers, for instance. Although you would plot them on the same scale to show how many, it would make no sense to join them up because there is nothing halfway between a fox and a badger, or a badger and a hedgehog.. . Similarly if you are comparing different days profit for a shop, it takes nothing in the middle of the night between Monday and Tuesday.... etc.