That chart is senseless propaganda. (And I fail to see how it is connected to the library of Alexandria?)

The lines are arbitrary (What do they measure? How do they measure it? What's the unit of "scientific advance"?) and the idea of a "dark age" during the Middle Ages has been shown to be false. To label it "christian dark age" is even more dishonest. If anything it was the transition from a large empire to many smaller kingdoms and other entities which was the cause for change (i.e. a social and political reason, not a religious one; and realise I'm making a sweeping generalized statement here which only virtue is that it is less wrong than saying religion is the cause).

See the FAQ for more information on the "dark ages".

Especially this question: Why are the European "Dark Ages" considered a misnomer?

And: How far do you believe we would have advanced as a civilization if the "Dark Ages" would never have happened?

You may be interested /u/cordis_melum 's response from over at /r/badhistory . Due to its popularity among r/atheist types, we like to just call it "The Chart."

I love the idea of 'the unit' of scientific advance.