How does difficulty impact profitability? what is the formula? I have an Antminer S9 ASIC miner , and I have visited this , but I want to know how difficulty may impact my profitability in the future. I would also like to estimate break even costs.
Source: http://bitcoin.sipa.be/speed-lin.png

The difficulty growth has been slowing down over the years, but in the last year it still has about quintupled.



Source: http://bitcoin.sipa.be/growth-10k.png

The general formula for calculating profitability is elaborated on How much Bitcoin will I mine right now with hardware X?.