Y axis range problems with Draw("same") and Draw("goff")

Hi everybody,

in my PyROOT script, I have a function to compute a median for each x-bin of a 2d histogram and fill the result into its own histogram. The 2d histograms are the result of a i[/i] t.Draw().

Now I would like to superimpose several of these “median curves”. For each, I

  • t.Draw() with option “goff”

  • i[/i] hist2d = GetHistogram(), store the result

  • convert the histogram to my median curve i[/i] hist_medians = medians(hist2d)

  • and hist_medians.Draw(“same”)

The problem:

The second t.Draw() uses the y axis range as defined by the first hist_medians.Draw(). Because the range of this curve is rather limited, the second t.Draw() can have many entries in the y-underflow or overflow bins. Depending on how many entries then lie in these y-under-/overflow bins, this means that the second hist_medians.Draw() can be very wrong! (In the extreme case simply clipping the curve at the edge of the window, i.e. the range associated with the first curve.)

How do I fix this?

All the best,
Chris

is hist_medians only a pointer to hist2d or is it a copy of hist2d ?

No, it’s its own object. The “2d-to-1d converter” looks like this:

def convert_2d(hist,function): # To get X-axis range & labels, start out with ProjectionX hm=hist.ProjectionX() # For each X bin in the original hist for i in range(1,hist.GetNbinsX()+1): # Project to Y within that X bin, store in histogram hpy with appropriate title hpy=hist.ProjectionY(hist.GetTitle()+"_py%d"%i,i,i) # Calculate the median from this temporary hpy median=function(hpy) # Set the median as the value for the corresponding X bin in the curve histogram hm.SetBinContent(i,median) # Return this histogram return hm

And I have a dictionary where I store

Short update:

I started

  • explicitly creating new histograms TH2D(“name”,“title”,nbinsx,…)
  • and using Draw("…>>name")

instead of going

  • Draw("…>>name(nbinsx,…)")
  • t.GetHistogram()

and now all of the 1D histograms turn out fine.

The problem now seems to be that they still show up as lines being clipped to the lower edge of the Y axis range, as this range isn’t adjusted when you add another histogram with Draw(“same”).

TTree::Draw is really weird (to me) when making 2D plots. Apparently depending on whether you use “>>” to name the thing something other than the default name, it either makes a TGraph object or a TH2 object.

This thread has me being confused about it, and a few experts giving cryptic/1-line comments: TGraph saved to file shows up as TH2F

Jean-François