GetErrorYlow

GetErrorYlow is a member function used in the ROOT data analysis framework, typically with graphs that support asymmetric errors such as TGraphAsymmErrors. It returns the lower (negative) magnitude of the Y error for the i-th data point.

The function is usually defined as Double_t GetErrorYlow(Int_t i) const, with i ranging from 0 to GetN()−1, reflecting ROOT’s zero-based indexing. If a graph uses symmetric Y errors, the lower and upper errors may be equal, and GetErrorYlow would match the corresponding upper value.

GetErrorYlow is part of a set of methods for accessing asymmetric error components on data points. It is used in conjunction with GetErrorYhigh to retrieve the upper Y error, and with GetErrorXlow / GetErrorXhigh for the X-direction errors. Per-point errors are commonly set via methods like SetPointError, which can specify eyl (lower Y error) and eyh (upper Y error) values for each point.

In practice, GetErrorYlow is useful for plotting accurate error bars and for downstream analysis that requires knowledge of the lower bound of the Y uncertainty. It is specific to graph classes that store asymmetric errors; for graphs with only symmetric Y errors, a separate GetErrorYlow may not be applicable.

See also: GetErrorYhigh, GetErrorXlow, GetErrorXhigh, SetPointError, TGraphAsymmErrors.