How can I plot a histogram such that the heights of the bars sum to 1 in matplotlib

Question

I d like to plot a normalized histogram from a vector using matplotlib   I tried the following   plt hist myarray  normed True    as well as   plt hist myarray  normed 1    but neither option produces a y-axis from  0  1  such that the bar heights of the histogram sum to 1  I d like to produce such a histogram -- how can I do it

User · Answer

If you want the sum of all bars to be equal unity, weight each bin by the total number of values:

weights = np.ones_like(myarray) / len(myarray)
plt.hist(myarray, weights=weights)

Hope that helps, although the thread is quite old...

Note for Python 2.x: add casting to float() for one of the operators of the division as otherwise you would end up with zeros due to integer division

User · Answer

Check this out  plt hist myarray  density   True

User · Answer

I know this answer is too late considering the question is dated 2010 but I came across this question as I was facing a similar problem myself  As already stated in the answer  normed True means that the total area under the histogram is equal to 1 but the sum of heights is not equal to 1  However  I wanted to  for convenience of physical interpretation of a histogram  make one with sum of heights equal to 1    I found a hint in the following question - Python  Histogram with area normalized to something other than 1  But I was not able to find a way of making bars mimic the histtype  step  feature hist    This diverted me to   Matplotlib - Stepped histogram with already binned data   If the community finds it acceptable I should like to put forth a solution which synthesises ideas from both the above posts   import matplotlib pyplot as plt    Let X be the array whose histogram needs to be plotted  nx  xbins  ptchs   plt hist X  bins 20  plt clf     Get rid of this histogram since not the one we want   nx frac   nx float len nx     Each bin divided by total number of objects  width   xbins 1  - xbins 0    Width of each bin  x   np ravel zip xbins  -1   xbins  -1  width   y   np ravel zip nx frac nx frac    plt plot x y linestyle  dashed  label  MyLabel        Further formatting    This has worked wonderfully for me though in some cases I have noticed that the left most  bar  or the right most  bar  of the histogram does not close down by touching the lowest point of the Y-axis  In such a case adding an element 0 at the begging or the end of y achieved the necessary result   Just thought I d share my experience  Thank you

User · Answer

It would be more helpful if you posed a more complete working  or in this case non-working  example   I tried the following   import numpy as np import matplotlib pyplot as plt  x   np random randn 1000   fig   plt figure   ax   fig add subplot 111  n  bins  rectangles   ax hist x  50  density True  fig canvas draw   plt show     This will indeed produce a bar-chart histogram with a y-axis that goes from  0 1    Further  as per the hist documentation  i e  ax hist  from ipython   I think the sum is fine too    normed   If  True   the first element of the return tuple will be the counts normalized to form a probability density  i e     n  len x  dbin      In a probability density  the integral of the histogram should be 1  you can verify that with a trapezoidal integration of the probability density function        pdf  bins  patches   ax hist          print np sum pdf   np diff bins     Giving this a try after the commands above   np sum n   np diff bins     I get a return value of 1 0 as expected   Remember that normed True doesn t mean that the sum of the value at each bar will be unity  but rather than the integral over the bars is unity   In my case np sum n  returned approx 7 2767

User · Answer

Here is another simple solution using np histogram   method   myarray   np random random 100  results  edges   np histogram myarray  normed True  binWidth   edges 1  - edges 0  plt bar edges  -1   results binWidth  binWidth    You can indeed check that the total sums up to 1 with    gt  print sum results binWidth  1 0

[python] How can I plot a histogram such that the heights of the bars sum to 1 in matplotlib?

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