IMO, the easiest way is to have a range on the dashboard (hidden if you don't want to see it, or put it on another tab) that pulls the desired data into a single block. Exactly how to build this range will depend on a number of factors. For simplicity, if this was the only time I was going to build this (as a template), I would do something like:
in dashboard A1:Bn:
Then use that range to build the chart.
Another thought, I don't know if it would apply. If your source data were in a better database format, I could see a simple pivot table/chart doing the same thing. Unless there is some other reason that you MUST store the data by month like your sample shows, I think there could be many advantages to consider storing the data in a good database arrangement and pivoting off of that to get the monthly summaries and the overall summary that you are looking for.
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