I think that you're misusing Chebyshev (do you really only want to give "3" as a coefficient..which is just the constant function 3), and you have to evaluate it in order to give matplotlib some x and y data to plot.

from matplotlib import pyplot as plt
import numpy as np

x = np.linspace(-1.0,1.0)
test = np.polynomial.Chebyshev((1,2,3))
y = test(x)

On Tue, Jan 10, 2012 at 9:10 AM, Fabien Lafont <lafont.fabien@gmail.com> wrote:
I'm trying to plot Chebyshev polynolmials using: numpy.polynomial.Chebyshev:

import math
from numpy import *
from numpy import polynomial as pol
from pylab import *
from scipy import *
from scipy import optimize
import warnings
warnings.simplefilter('ignore', np.RankWarning)

test = pol.Chebyshev(3)
print test
plot (test)


The print return:      cheb([ 3.])

and plot :

Traceback (most recent call last):
 File "T:\Dropbox\Thèse\Python\fit sonde\test_poly_Tcheb.py", line
32, in <module>
   plot (test)
 File "C:\Python27\lib\site-packages\matplotlib\pyplot.py", line 2458, in plot
   ret = ax.plot(*args, **kwargs)
 File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 3849, in plot
 File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 1443,
in add_line
 File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 1451,
in _update_line_limits
   p = line.get_path()
 File "C:\Python27\lib\site-packages\matplotlib\lines.py", line 644,
in get_path
 File "C:\Python27\lib\site-packages\matplotlib\lines.py", line 401, in recache
   y = np.asarray(yconv, np.float_)
 File "C:\Python27\lib\site-packages\numpy\core\numeric.py", line
235, in asarray
   return array(a, dtype, copy=False, order=order)
TypeError: float() argument must be a string or a number

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