This graph is one of the reasons I'm getting more skeptical about the Global Warming claims. Look at the lines in the graph. The bottom lines clearly show what used to be known as the "Medieval Warm Period" and the "Little Ice Age." The top lines show what people call the "Hockey Stick." In the run-up to the present, all the lines converge on a sharp up-slope.
If all the data is equally good, it makes sense to average things out and get a mild roller-coaster effect. If some of the data is unreliable, it makes sense to throw it out. I'm troubled by the possibility that researchers are motivated to find new lines of evidence that "eliminate" the Medieval Warm Period, but are not equally motivated to find evidence that confirms it. I'm suspicious that journals and grant agencies may call evidence of the MWP "old news," unworthy of grants or print, while evidence against it is "hot" and deserves fast-track treatment.