Wiki:
"The Medieval Warm Period (MWP) or Medieval Climate Optimum was a time of unusually warm climate in the North Atlantic region, lasting from about the tenth century to about the fourteenth century."
...
"It was initially believed that the temperature changes were global [2]. However, this view has been questioned; the 2001 IPCC report summarises this research, saying "…current evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries"
http://en.wikipedia.org/wiki/Medieval_Warm_Period

This has been shown to be wrong.

Btw, your graph has this caption:
"Northern Hemisphere temperature reconstructions for the past 2000 years."

Of course there is also historical evidence of the Medieval Warm Period in Asia, so you are correct in saying this was already known, but there has been a campaign to spin that Medieval Warm Period and Little Ice Age as being only regional climate variations- based on the "data" of the Hockey Stick. And there are still some [including the IPCC, especially the author:)] clinging to the Hockey Stick even though it has been shown not to be scientifically based [it's pseudo science- proven without any doubt as such, by Mann's refusal to have his work repeated by anyone else who lives on this planet].