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

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.)

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

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.