It seems you're getting bitten by the brain-dead interpolation of points
(which is meant to draw line segments as curves that follow the radii).
I have fixed the polar drawing code in SVN r6106 to normalize theta to
(0.0 <= theta <= 2pi) before doing the interpolation, which seems to fix
your example. The user doesn't/shouldn't have to care about whether
theta is negative.
Cheers,
Mike
jan gillis wrote:
> Hi Tony,
>
> Thank you for the reply, the solutions you propose are fine in this
> case. But I'm trying to use the polar plot
> as a smith chart for an instrument and there i will receive data that is
> unknown but can be something like this:
>
> r = np.transpose(.1+np.arange ( 0 , 0.7 , 0.001))
> theta = -4.5 * np.pi *r
> freq = r*10e9
> data = np.multiply(r,np.exp(1j*theta))
> ax.plot(angle(data),abs(data))
>
> Any idea why Polar plot can't handle theta going from negative to
> positive radians?
>
> Jan
>
> Tony S Yu wrote:
>
>> On Sep 17, 2008, at 1:59 AM, jan gillis wrote:
>>
>>
>>> Hello,
>>>
>>> I have a problem with polar plot, if i run the following code in
>>> matplotlib 0.98.3, polar plot is drawing a extra circle to go from
>>> angle -3.14159265 to angle 3.03753126. Is there a solution for this
>>> problem?
>>>
>>> ********************
>>> import numpy as np
>>> from matplotlib.pyplot import figure, show, rc, grid
>>>
>>> # radar green, solid grid lines
>>> rc('grid', color='#316931', linewidth=1, linestyle='-')
>>> rc('xtick', labelsize=15)
>>> rc('ytick', labelsize=15)
>>>
>>> # force square figure and square axes looks better for polar, IMO
>>> fig = figure(figsize=(8,8))
>>> ax = fig.add_axes([0.1, 0.1, 0.8, 0.8], polar=True, axisbg='#d5de9c')
>>>
>>> z = np.zeros((1,2000),complex)
>>> z.real = 0.2
>>> z.imag = np.arange(-50,50,0.05)
>>> gamma_r = np.transpose((z-1)/(z+1))
>>>
>>> ax.plot(np.angle(gamma_r), np.abs(gamma_r), '.-', zorder=0)
>>>
>> Hi Jan,
>>
>> It looks like you get the circle because the angles you're plotting go
>> from negative to positive radians in a weird way. The circle being
>> drawn starts around 0 radians and goes clockwise by negative values.
>> Then when it gets to - pi, it switches to positive indices, i.e. pi.
>> Of course, these are the same points on a polar plot, but different
>> angles, if you want to be consistent.
>>
>> Here are a couple of quick solutions, but there but there maybe better
>> ways of handling this.
>>
>> # ~~~~~~~~~~~~~~~~~~
>> # get rid of the plot line above, and add the following
>> theta = np.angle(gamma_r)
>> mag = np.abs(gamma_r)
>>
>> # option 1
>> ordered = np.argsort(theta, axis=0).squeeze()
>> ax.plot(theta[ordered], mag[ordered], '.-', zorder=0)
>>
>> # option 2
>> neg_theta = np.where(theta < 0)
>> theta[neg_theta] += 2 * np.pi
>> ax.plot(theta, mag, '.-', zorder=0)
>> # ~~~~~~~~~~~~~~~~~~
>>
>> I hope that's helpful,
>> -Tony
>>
>>
>>> ax.set_rmax(2.0)
>>> grid(True)
>>>
>>> show()
>>>
>>> ********************
>>> Kind regards,
>>> Jean
>>>
>>>
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>>
>
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--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA
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