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Local Sensitivity Graph Notes
For local sensitivity design studies, if you gave your design variable an especially large range, a graph can cross the X or Y axis and look as if it were showing negative values.
This does not mean that there are negative values associated with the graph. It may appear this way in a local sensitivity study because the result of interest is the slope of the curve between two sample points, not specific values.
For more information about local sensitivity graphs, see Local Sensitivity Study.