How to fit a TGraphErrors?

TGraphErrors can define x errors, how to write a fcn to fit it?

When you call TGraphErrors::Fit (ie TGraph::Fit) the errors in x and y
are taken into account. See function GraphFitChisquare in TFitter

// In case of a TGraphErrors object, ex, the error along x, is projected // along the y-direction by calculating the function at the points x-exlow and // x+exhigh. // // The chisquare is computed as the sum of the quantity below at each point: // // (y - f(x))**2 // ----------------------------------- // ey**2 + ((f(x+exhigh) - f(x-exlow))/2)**2 // // where x and y are the point coordinates. // // In case the function lies below (above) the data point, ey is ey_low (ey_high).