The graph is also a variant of the Mann "Hockey Stick" graph which has been discredited.
Here are those papers that show that the graph is based on flimsy methodology:
Von Storch, Science, 2004
McIntyre & McKitrick, Geophysical Research Letters, 2005
Mangini et al, Earth and Planetary Science Letters, 2005.
Plus there are two other reports that vastly condemn the graph, commonly known as the “North” report, and the Wegman report. The first was done by an arm of the National Academy of Sciences, the second was done by a panel of statisticians and others for a Congressional committee.