Draw samples from a chisquare distribution.
When df independent random variables, each with standard normal distributions (mean 0, variance 1), are squared and summed, the resulting distribution is chisquare (see Notes). This distribution is often used in hypothesis testing.
Parameters :  df : int
size : tuple of ints, int, optional


Returns :  output : ndarray

Raises :  ValueError :

Notes
The variable obtained by summing the squares of df independent, standard normally distributed random variables:
is chisquare distributed, denoted
The probability density function of the chisquared distribution is
where is the gamma function,
References
[R163]  NIST/SEMATECH eHandbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook/eda/section3/eda3666.htm 
[R164]  Wikipedia, “Chisquare distribution”, http://en.wikipedia.org/wiki/Chisquare_distribution 
Examples
>>> np.random.chisquare(2,4)
array([ 1.89920014, 9.00867716, 3.13710533, 5.62318272])