Samsung's S8 sales were less than the S7. Might be telling for Apple since they basically will be presenting their version of the same product. Bezelless design, facial recognition/iris scanning camera, etc.That chart looks pretty flat since the start of FY 15. The only thing sustaining the limited revenue growth during that time is the increase in average selling price of the iPhone as more people get either the plus model or opt for larger storage. However, Apple will eventually run up against a ceiling where people won't keep piling on the options and will face increased competition from other vendors.
Apple's outstanding growth has been in the perfection of markets for broad consumer adoption. It started in 2001 with the iPod, really hits its stride with the iPhone in 2007 which managed seven years of continuous market growth until the iPhone 6 in 2014, and was helped by the iPad in 2010 which had a few years of exponential growth before plateauing.
Apple's problem now is the unit sales of their main products have been stagnant for three years and they don't have a revolutionary product on the horizon. This year's iPhone needs to have 20% or better unit sales growth to keep Apple as a leading tech company. If sales remain flat they will certainly continue to be successful but will face a transition to a commodity provider as opposed to the market leader they have been over the last 16 years.
The only difference is apple has released an iPhone 6s and 6ss the last couple years, so, as analysts have said, there is "pent up demand" for this year's redesign.