That's comparing apples to oranges. All the other data used to create the original graph o calculate temperature such as tree rings, etc do not take into account the temperature of the oceans, etc. There may be perfectly good explanations as to why the temperature may be flat, but those same explanations are not being fed into the graph that the OP posted.

>All the other data used to create the original graph o calculate temperature such as tree rings, etc do not take into account the temperature of the oceans, etc

https://en.wikipedia.org/wiki/Proxy_(climate)#Corals