TGraphAsymmErrors

TGraphAsymmErrors is a class in the ROOT data analysis framework designed to represent a graph with asymmetric error bars on each data point. It stores, for every point, the coordinates x[i] and y[i], along with four error values: exl[i] and exh[i] for the left and right x-errors, and eyl[i] and eyh[i] for the lower and upper y-errors.

The class derives from TGraph and extends it with per-point asymmetric uncertainties. Points are added or modified with SetPoint, and the per-point uncertainties are assigned with SetPointError(i, exl, exh, eyl, eyh). Accessors such as GetPoint and the GetErrorXlow, GetErrorXhigh, GetErrorYlow, and GetErrorYhigh methods return the coordinate values and the corresponding asymmetric errors for a given point.

Usage typically mirrors that of other ROOT graph classes, but with the ability to specify different error magnitudes on each side of a point. When drawn, the graph displays error bars that reflect the stored exl, exh, eyl, and eyh values, offering a more accurate representation of measurements with non-symmetric uncertainties. The class supports standard graph operations such as copying, clearing, and removing points, inherited from the base graph type.

TGraphAsymmErrors is contrasted with TGraphErrors, which stores only symmetric errors. It is particularly useful in analyses where systematic or statistical uncertainties differ on the left and right sides of an x-value or up and down in y. It is commonly used in high-energy physics and other fields requiring precise representation of asymmetric uncertainties in plotted data.