The intermittently good blog Junk Charts has another post highlighting the danger of thinking that you can analyse the relationship between two variables simply by plotting them next to each other on a line chart.
The problem is that line charts really don't give you the ability to tell if there is a relationship or not, but your brain may well con you into believing that they do because of our tendency to see patterns (even if they're not there). Take this chart:
Is there a relationship between the two lines? When you've made up your mind click here to see a scatter plot of the same data. It's a lot easier to tell isn't it?
Why are people so afraid of scatterplots? They're one of the most useful basic tools in any analyst's toolkit, and they're really a very simple idea—essentially a scatterplot is just a map. If you don't feel comfortable with them perhaps it's time you learnt to be!