In the absence of other information, it's reasonable to assume linear scaling.

You haven't seen many charts, right?

Part of my training for a previous job was how to create presentations, including how to create charts to make numbers look good even if they aren't. To get an idea of some basic techniques, search for lie with charts. Some of the results:

• A Quick Guide to Spotting Graphics That Lie (needs a lot of Javascript)
• How to lie with charts
• Axes of evil: How to lie with graphs
• Five Ways to Lie with Charts
• Vision Statement: How to Lie with Charts (needs a lot of Javascript)

Alexander