data type and logarithm

[simona bellavista]

Hi, I am quite new to python and I am trying to do some simple plots.
I am using python Python 2.6.4 and numpy/1.5.1
I have an ASCII  data file that I am reading with the following lines
of code:

import pylab
import numpy as np

filename='something.dat'
file = open(filename)

rho = np.array([], dtype = 'float64')
entropy = np.array([], dtype = 'float64')
for line in file:
    columns = line.split()
    rho = np.append(rho,columns[0])
    entropy = np.append(entropy,columns[1])

and rho and entropy are apparently read correctly, but when I look to
the data type

print rho.dtype
print entropy.dtype

I get  |S22 , what's that?
Then I want to plot a logarithmic plot and I do

pylab.plot(np.log(rho), entropy)

and I get

NotImplementedError: Not implemented for this type

Does anybody has a clue? I do not know how to proceed

Many thanks