If the tree-ring growth data doesn't match directly measured temperature at significant intervals of time (necessitating what amounts to arbitrarily changing data sources mid graph) why on earth would you trust it for periods where its the only data source.
I think the evidence for AGW extends beyond this particular incident, but I think its highly unethical and very poor science.
That's a fair criticism, but it would be worth seeing the figure caption for it as well. If the caption had within it that the source was switched part way due to a known problem, that would be fine.
Out of curiosity, anyone know a journal reference for that particular graph?
Comments on the OP suggest it was cover art. Not sure what to make of that without more context.
Making it clear the data changed stops it being unethical, but I think the criticism of concatenating two data sources remains. Especially when the point of the graph is the sharp rise at the end - just as the data source changes. You really want to make sure the data sources correlate highly to make that kind of argument.
I think the evidence for AGW extends beyond this particular incident, but I think its highly unethical and very poor science.