TGraphErrors

TGraphErrors is a class from the CERN ROOT data analysis framework that represents a two‑dimensional graph with error bars. It extends the basic TGraph class by storing, for each point, the associated uncertainties in the X and Y coordinates. A TGraphErrors object is typically created by providing arrays of X and Y values together with their errors, e.g. \(x[i], y[i], ex[i], ey[i]\), and a number of points. Its constructor can also accept vectors or std:: arrays, allowing flexible data input.

Once instantiated, the graph can be drawn with the Draw() method, using options such as "AP" for axis and points or "e" to display error bars only. Compared to the simple TGraph, TGraphErrors automatically handles the display of asymmetrical errors if provided. The class exposes methods to modify individual points and their errors after construction, which is useful for iterative fitting procedures.

Data contained in a TGraphErrors can be accessed via GetPoint and GetError methods; GetPoint returns the (x,y) value for a given index, while GetError retrieves the corresponding ex or ey. The number of points can be queried with GetN. Because ROOT is written in C++, the class can be used from C++ code, from Python via PyROOT, or from batch scripts that automate analysis.

TGraphErrors is commonly used in high‑energy physics to visualize measurement points with uncertainties, to feed into fitting functions that take errors into account, and to publish results where the error bars convey statistical or systematic uncertainties. Its integration with ROOT’s other classes, such as TF1 for fitting or TCanvas for layout, makes it a standard tool for scientific charting in experimental physics.