Legendary fund manager Peter Lynch once said that you shouldn't invest in any idea you couldn't illustrate with a crayon.

Though I'm not much for crayons, I do love the pithiness of that line. We regularly preach the same idea at the Fool: Don't buy what you don't understand. And if you can't simply sketch out a company's business model -- how it actually makes money -- then maybe you shouldn't be investing in it.

As a Ford (NYSE: F) shareholder, I can attest to the fact that it's been a tough year. I've seen more and more Fords on the road, but the share price hasn't reflected that observation the way I would have hoped. Let's take a graphical look at Ford's revenue performance over the past three years and see whether we can figure out why the stock price might be lagging in the face of strong revenue growth domestically. (Note: I'm focusing on revenues here, not earnings.)

Source: Ford 2011 10-K.

Think I missed something in this illustration? General thoughts on this exercise? Let me know in the comments section below. And if you haven't already, be sure to follow our Ford news and commentary using the Fool's free new My Watchlist tool.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.