Did the iPod's design evolution influence Apple's stock price?

The Wall Street Journal just published an interesting graph overlaying different iPod releases with Apple's stock price: 

Core77 Design Blog points to this as proof that good design can influence stock price.  This is what they say:

The clarity of their cohesive design language and its evolution over time is also instantly appreciated at a glance. If ever business needed confirmation that design can have a significant effect on your ROI, then this piece of infoporn is it.

At first glance I'd love to agree right off the bat, because I am a strong believer in the ROI of good design.  However, the researcher in me has to take a step back and ask if we can really say this based off this one chart.  As every researcher will tell you, correlation does not imply causality.  Just because 2 things happen at the same time, it does not mean that one event causes the other. 

Forgive me for stating the obvious, but there are hundreds of factors that influence the stock price, and we simply can't prove causality just by looking at this graph.  For example, how do we use design to explain the sudden drop in stock price between the iPod video launch and the redesign iPod shuffle launch?

Having said that, I do think that this poses an interesting question about what effect good user experience has on business metrics and revenue.  I guess if we can find a way to prove that beyond the shadow of a doubt, we won't have to spend half our time convincing people that good user experience is important...