The fast-casual chain is still firing on all cylinders.Chipotle Mexican Grill (CMG -0.52%) stock has rallied about 340% over the past five years as the S&P 500 rose 85%. The fast-casual restaurant chain repeatedly impressed investors with its robust growth, consistent expansion, and rising margins.Today, the stock may seem expensive at 57 times forward earnings estimates, but a look at its growth story through four simple charts is enough to see if it's still worth buying. 1. Consistent restaurant openingsU ...