This tutorial demonstrates the use of TGraphAsymmErrors to plot a graph with asymmetrical errors on both the x and y axes.
The errors for the x values are divided into low (left side of the marker) and high (right side of the marker) errors. Similarly, for the y values, there are low (lower side of the marker) and high (upper side of the marker) errors.
Definition in file gr004_errors_asym.C.