doc/v626/rf609__xychi2fit_8py.html

import ROOT

import math


# Create dataset with X and Y values

# -------------------------------------------------------------------


# Make weighted XY dataset with asymmetric errors stored

# The StoreError() argument is essential as it makes

# the dataset store the error in addition to the values

# of the observables. If errors on one or more observables

# are asymmetric, can store the asymmetric error

# using the StoreAsymError() argument


x = ROOT.RooRealVar("x", "x", -11, 11)

y = ROOT.RooRealVar("y", "y", -10, 200)

dxy = ROOT.RooDataSet("dxy", "dxy", {x, y}, StoreError={x, y})


# Fill an example dataset with X,err(X),Y,err(Y) values

for i in range(10):

    x.setVal(-10 + 2 * i)

    x.setError((0.5 / 1.0) if (i < 5) else (1.0 / 1.0))


    # Set Y value and error

    y.setVal(x.getVal() * x.getVal() + 4 * abs(ROOT.gRandom.Gaus()))

    y.setError(math.sqrt(y.getVal()))


    dxy.add({x, y})


# Perform chi2 fit to X +/- dX and Y +/- dY values

# ---------------------------------------------------------------------------------------


# Make fit function

a = ROOT.RooRealVar("a", "a", 0.0, -10, 10)

b = ROOT.RooRealVar("b", "b", 0.0, -100, 100)

f = ROOT.RooPolyVar("f", "f", x, [b, a, 1.0])


# Plot dataset in X-Y interpretation

frame = x.frame(Title="Chi^2 fit of function set of (X#pmdX,Y#pmdY) values")

dxy.plotOnXY(frame, YVar=y)


# Fit chi^2 using X and Y errors

f.chi2FitTo(dxy, YVar=y)


# Overlay fitted function

f.plotOn(frame)


# Alternative: fit chi^2 integrating f(x) over ranges defined by X errors, rather

# than taking point at center of bin

f.chi2FitTo(dxy, YVar=y, Integrate=True)


# Overlay alternate fit result

f.plotOn(frame, LineStyle="--", LineColor="r")


# Draw the plot on a canvas

c = ROOT.TCanvas("rf609_xychi2fit", "rf609_xychi2fit", 600, 600)

ROOT.gPad.SetLeftMargin(0.15)

frame.GetYaxis().SetTitleOffset(1.4)

frame.Draw()


c.SaveAs("rf609_xychi2fit.png")