The core of the problem is that (s)he wants to rebin a “fix bin sizes” 2-D histogram into a “variable bin sizes” histogram. Current ROOT (up to and including 6.10.04) supports it only for 1-D histograms (i.e. not for 2-D, neither for 3-D histograms -> I guess you could put it on your “to-do” list).
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