def canvas(name='canvas1', size=(800, 600)):
    """
    Helper method for creating canvas
    If canvas with this name already exists, it will be returned
    """
    import ROOT
    canvas = ROOT.gROOT.FindObject(name)
    if canvas:
        return canvas
    else:
        width, height = size
        return ROOT.TCanvas(name, name, width, height)
