You have begun with the incorrect assumption that your bitcoin miner will hash at a constant percentage of the network hashing rate for 730 days. I fail to see the usefulness of your graphs.
It makes no such assumption. As long as the network hashrate is at or below the lines on the chart, at any point in time you could buy a miner and end up breaking even after 2 years, of course constrained by the listed assumptions and the curves themselves, nothing else. If your point is that after 2 years the network may exceed those lines, thats actually part of the point, but it requires either changing constraints or someone betting > 2 year.
Let's dissect the data where all points converge @1239PH/s network speed and no cost for electricity.
1 TH/s is costing $700 now on your chart and bitcoin is worth $330;
1239 PH/s = 1239000 TH/s
Take the reciprocal of this 1/1239000 to find out your share of the block rewards for 1 TH/s and multiply that by the total number of bitcoins produced in a day.
total bitcoins per day for the entire network (ideal):
25 per block (reward) x 6 (blocks per hour) x 24 hours per day = 3600
3600 multiplied by 1/1239000 = 0.0029055690072639225181598062954 Bitcoins per day (ideal)
Now if you make that every single day for the magical number of 730 days you get 2.121065375302663438256658595642 bitcoins
and when we multiply that by the assumed $330 per bitcoin... drumroll please... we get $699.95157384987893462469733656186 or approximately $700 dollars for our 1 TH/s miner that we paid $700 dollars for 730 days ago! Which only works for a sustained network hashing rate of 1239 PH/s for a period of 730 days.
Your graph and your assumptions are flawed.
^that is my point
I think you're missing the point of the graph. It's purpose is to illustrate, given certain parameters, what the final steady state network hashrate could end up being. Barring large changes in price or available technology, eventually the network will get to the point that even a large operation in a location with extremely cheap electricity will not be able to break even in X days even without difficulty changing. In that case, investment in mining hardware will slow significantly. It will probably continue to grow as ASICs are useless for anything else and manufacturers will probably sell what they have in stock or in process for whatever they can recover, but without better hardware or an exchange rate swing eventually the network hashrate will stop growing.