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Hello,
I'm using mpl_toolkits.axisartist.floating_axes.GridHelperCurveLinear
for creating half-polar plots from 180 degree measurements for receive 
sensitivity.

Working with the measurement values itself is no problem if I let the 
values scaling start at zero.
If I use normalized values I can plot it also, but if I transform it 
into the dB scale I got a segfault in this lib.

I provide an example. For my problems I would like to have a solution 
that I can either use r limit from -30 to 0 (f3) or changing the tick 
labels in figure f2.

And by the way is there a possibility that the if i want to plot data in 
the range from 80 to 120, that rlim(80,120) would set the 80 to the 
centerpoint? At the moment I got only a small stripe.

[code]
"""Demo of polar plot of arbitrary theta. This is a workaround for MPL's 
polar plot limitation
to a full 360 deg.

Based on 
http://matplotlib.org/mpl_toolkits/axes_grid/examples/demo_floating_axes.py

get from 
https://github.com/neuropy/neuropy/blob/master/neuropy/scripts/polar_demo.py
TODO: license / copyright
"""

from __future__ import division
from __future__ import print_function

import numpy as np
import matplotlib.pyplot as plt

from matplotlib.transforms import Affine2D
from matplotlib.projections import PolarAxes
from mpl_toolkits.axisartist import angle_helper
from mpl_toolkits.axisartist.grid_finder import MaxNLocator
from mpl_toolkits.axisartist.floating_axes import GridHelperCurveLinear, 
FloatingSubplot


def fractional_polar_axes(f, thlim=(0, 180), rlim=(0, 1), step=(30, 0.2),
                           thlabel='theta', rlabel='r', ticklabels=True, 
theta_offset=0):
     """Return polar axes that adhere to desired theta (in deg) and r 
limits. steps for theta
     and r are really just hints for the locators."""
     th0, th1 = thlim # deg
     r0, r1 = rlim
     thstep, rstep = step

     tr_rotate = Affine2D().translate(theta_offset, 0)
     # scale degrees to radians:
     tr_scale = Affine2D().scale(np.pi/180., 1.)
     #pa = axes(polar="true") # Create a polar axis
     pa = PolarAxes
     tr = tr_rotate + tr_scale + pa.PolarTransform()
     theta_grid_locator = angle_helper.LocatorDMS((th1-th0)//thstep)
     r_grid_locator = MaxNLocator((r1-r0)//rstep)
     theta_tick_formatter = angle_helper.FormatterDMS()

     grid_helper = GridHelperCurveLinear(tr,
                                         extremes=(th0, th1, r0, r1),
                                         grid_locator1=theta_grid_locator,
                                         grid_locator2=r_grid_locator,
 
tick_formatter1=theta_tick_formatter,
                                         tick_formatter2=None)

     a = FloatingSubplot(f, 111, grid_helper=grid_helper)

     f.add_subplot(a)

     # adjust x axis (theta):
     a.axis["bottom"].set_visible(False)
     a.axis["top"].set_axis_direction("bottom") # tick direction
     a.axis["top"].toggle(ticklabels=ticklabels, label=bool(thlabel))
     a.axis["top"].major_ticklabels.set_axis_direction("top")
     a.axis["top"].label.set_axis_direction("top")

     # adjust y axis (r):
     a.axis["left"].set_axis_direction("bottom") # tick direction
     a.axis["right"].set_axis_direction("top") # tick direction
     a.axis["left"].toggle(ticklabels=ticklabels, label=bool(rlabel))

     # add labels:
     a.axis["top"].label.set_text(thlabel)
     a.axis["left"].label.set_text(rlabel)

     # create a parasite axes whose transData is theta, r:
     auxa = a.get_aux_axes(tr)
     # make aux_ax to have a clip path as in a?:
     auxa.patch = a.patch
     # this has a side effect that the patch is drawn twice, and 
possibly over some other
     # artists. So, we decrease the zorder a bit to prevent this:
     a.patch.zorder = -2


     # add sector lines for both dimensions:
     thticks = grid_helper.grid_info['lon_info'][0]
     rticks = grid_helper.grid_info['lat_info'][0]
     for th in thticks[1:-1]: # all but the first and last
         auxa.plot([th, th], [r0, r1], '--', c='grey', zorder=-1)
     for ri, r in enumerate(rticks):
         # plot first r line as axes border in solid black only if it 
isn't at r=0
         if ri == 0 and r != 0:
             ls, lw, color = 'solid', 2, 'black'
         else:
             ls, lw, color = 'dashed', 1, 'grey'
         # From http://stackoverflow.com/a/19828753/2020363
         auxa.add_artist(plt.Circle([0, 0], radius=r, ls=ls, lw=lw, 
color=color, fill=False,
                         transform=auxa.transData._b, zorder=-1))

     return auxa


if __name__ == '__main__':
     f1 = plt.figure(facecolor='white', figsize=(16/2.54, 12/2.54), dpi=600)
     a1 = fractional_polar_axes(f1, thlim=(-90, 90),step=(10, 
0.2),theta_offset=90)
     # example spiral plot:
     thstep = 10
     th = np.arange(-90, 90+thstep, thstep) # deg
     rstep = 1/(len(th)-1)
     r = np.arange(0, 1+rstep, rstep)
     a1.plot(th, r, 'b')


     f1.show()

     f2 = plt.figure(facecolor='white', figsize=(16/2.54, 12/2.54), dpi=600)
     a2 = fractional_polar_axes(f2, thlim=(-90, 
90),rlim=(0,30),step=(10, 8),theta_offset=90)
     # example spiral plot:
     r2 = 20 * np.log10(r) +30
     a2.plot(th, r2, 'b')

     f2.show()

     f3 = plt.figure(facecolor='white', figsize=(16/2.54, 12/2.54), dpi=600)
     a3 = fractional_polar_axes(f2, thlim=(-90, 90),rlim=(-30,0), 
step=(10, 8),theta_offset=90)
     # example spiral plot:
     r3 = 20 * np.log10(r)
     a3.plot(th, r2, 'b')

     f2.show()

[\code]

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