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. That graph shows an estimate of a total of about 500 ZJ added to the the upper 2 km of the oceans since 1955.
Aside: Actually, the first 50 years of the graph are nothing but guesses.
OHC is estimated by models, informed by temperature measurements, made by Argo floats. The first Argo float was deployed in 2000. They didn't reach 3000 units (i.e., one float per 120,000 kmยฒ) until 2007.
So the part of their graph prior to about 2005 is 100% codswallop. The kindest thing you can say about it is that it's a plausible guess, consistent with (but you can't say based upon!) convenience samples of sea surface temperatures. It is not data, in any sense.
But never mind that, because that graph also has another problem...