The General Histogram VI allows for min and max tolerances and will output the number of hits outside of that range. You could use that VI in a while loop and wire the out-of-range values to shift registers, along with an "Add" VI, to create a running sum. You'd need to do this for your X & Y axis.
Also, for an example, the Running Histogram Example shows a use of the General Historgram vi output, buts converts the out-of-range values to % versus a running sum.
However, as you also thought, using simple indicators along with the "In Range and Coerce" VI (Functions > Comparison) wired with the graph data input (X and Y separately) is another method that would fit just as well, and may be easier to implement.
I believe either way will work fine. Good luck with it, Do
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