What happened to the average blocktime during this period?

Question

What happened to Ethereums blocktime from April to October last year (2017)?

In the whitepaper, Vitalik said the difficulty should adjust at a maximum rate of 0.1% per block (around 100% per 12 hours). Is this not the case nowadays?

Answer 1

This was the Difficulty Bomb, originally put in place in the lead up to Proof of Stake being implemented. The aim was to make mining gradually more difficult before the switchover.

See: What is the "difficulty bomb" and what is the goal of it?

As part of EIP-669, the Difficulty Bomb was delayed, which is why your graph returns to normal levels.