Limits plot with asymmetric -/+ 1 and 2 sigma bands

[ChilufyaM]

Continuing the discussion from Brazil-flag plot using TGraphAsymmError:

Hello, apologies for bringing back this topic. I would like to make a Brazilian flag plot and I’m wondering how the +/- 1 and 2 sigma values should be defined.
To follow from the example given in the linked previous post, I’m wondering if, for example, -1 sigma limits should be in ex, and +1 sigma limits in ey.

Hope the question makes sense.

Thanks for your help,
Chilufya

[Danilo]

Hi Chilufya,

Have you tried to have two graphs, with the same points but two different sets of errors along the y axis?

Best,
D

[ChilufyaM]

Hello @Danilo

Thanks for your reply. And I’m sorry for the noise. I figured out my issue by using TGraphAsymmErrors such that I can now pass my 1sigma values in one graph, and the 2 sigma values in another graph. Previously, I was using TGraphErrors which gives symmetric errors. Here’s a snippet of my python code in case it would be useful to someone else in future. The ROOT version used is v6-30-02.

from ROOT import TCanvas, TGraphErrors, TGraph, TGraphAsymmErrors
from ROOT import gROOT
from array import array
import math

canvas = TCanvas("c41","c41",200,10,600,400);
n = 5
#mass points                                                                            
x  = array( 'f', [800, 900, 1000, 2000, 3000])
#median limits                                                                          
y  = array( 'f', [0.14235, 0.155591, 0.172235, 0.203118, 0.254512])
ex = array( 'f', [0, 0, 0, 0, 0] )

#-1sigma limits                                                                         
ey1_minus = array( 'f', [math.sqrt(0.081054-0.058052)*0.5, math.sqrt(0.387335-0.274752)\
*0.25, math.sqrt(0.474636-0.334829)*0.25, math.sqrt(0.660109-0.471198)*0.25, math.sqrt(\
1.036419-0.736260)*0.25])

#+1sigma limits                                                                         
ey1_plus = array( 'f', [math.sqrt(-0.081054+0.114350)*0.5, math.sqrt(-0.387335+0.542341)*0.25, math.sqrt(0.662648-0.474636)*0.25, math.sqrt(0.924294-0.660109)*0.25, math.sqrt(1.463198-1.036419)*0.25])

#-2sigma:                                                                                
ey2_minus = array( 'f', [math.sqrt(0.081054-0.043290)*0.5, math.sqrt(0.387335-0.204179)*0.25, math.sqrt(0.474636-0.249315)*0.25, math.sqrt(0.660109-0.349909)*0.25, math.sqrt(1.036419-0.546009)*0.25])

#+2sigma                                                                                
ey2_plus = array( 'f', [math.sqrt(-0.081054+0.154670)*0.5, math.sqrt(-0.387335+0.733277)*0.25, math.sqrt(0.899110-0.474636)*0.25, math.sqrt(1.254989-0.660109)*0.25, math.sqrt(2.009040-1.036419)*0.25])

gr = TGraph( n, x, y )
gr.SetLineColor( 4 )
gr.SetLineWidth( 2 )
gr.SetMarkerColor( 4 )
gr.SetMarkerStyle( 20 )
gr.SetMarkerSize( 1 )

ge = TGraphAsymmErrors(n, x, y, ex, ex, ey2_minus, ey2_plus)
ge.SetFillColor(5)
ge.GetYaxis().SetRangeUser(-1.0, 1.0)
ge.GetXaxis().SetRangeUser(800, 3000)
ge.Draw("a3")

ge2 = TGraphAsymmErrors(n, x, y, ex, ex, ey1_minus, ey1_plus)
ge2.SetFillColor(3);
ge2.Draw("same3l")
#ge2.Draw("a3")                                                                         
gr.Draw( 'SameLP' )
canvas.SaveAs("limits.pdf")

Regards,
Chilufya

[Danilo]

Thanks for the code and positive attitude, Chilufya!