If the Bitcoin would have had a constant amount of hashing power that whole time, we could have expected the straight line that you postulated.
However, the Bitcoin network's hashing power has been growing substantially, for some stretches even exponentially.
As the difficulty on the Bitcoin network only gets adjusted every 2016 blocks, i.e. every 14 days at a 10 minute interval, the difficulty has only been in line with the amount of hashing power right after a difficulty change.
Yet, as the hashing power has been increasing, blocks were found quicker, reducing the average block time below the target of ten minutes. – It actually dropped below eight minutes at peak hashing power growth rates.
As each block puts new bitcoins into circulation, a high growth in hashing power will cause a greater increase of bitcoins into circulation. Therefore, the growth of hashing power is related to the slope of "Total Bitcoins in circulation".
On the other hand, the hash rate in 2015 so far has been decreasing from a peak of 328 Petahashes/s to about 300 Petahashes/s, causing a current average block interval of just shy of 11 minutes.