如何在matplotlib的极轴上使用对数刻度

Question

我正在尝试制作一个极坐标图，并且想在径向轴上使用对数刻度，但我总是遇到错误。下面是一些示例代码和错误信息。看起来在笛卡尔坐标系下是没问题的，有谁知道这是怎么回事吗？

import matplotlib.pyplot as plt
import numpy as np
import matplotlib.cm as cm

bazbins = np.linspace(0, 2*np.pi, 360)
fbins = np.logspace(np.log10(0.05), np.log10(0.5), 101)
theta, r = np.meshgrid(bazbins, fbins) 

# Set up plot window
fig, ax = plt.subplots(figsize=(12,9))#, subplot_kw=dict(projection='polar'))

# polar
ax.set_theta_zero_location('N')
ax.set_theta_direction(-1)
ax.set_rscale('log')

# carthesia
#ax.set_yscale('log')

# Plot data
#ax.pcolormesh(theta, r, r)
plt.gca().invert_yaxis()
ax.contourf(theta, r, r)
ax.set_ylim((0.0, 0.5))
plt.show()

在Tkinter回调中出现异常：

Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1413, in __call__
    return self.func(*args)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 236, in resize
    self.show()
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 239, in draw
    FigureCanvasAgg.draw(self)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 421, in draw
    self.figure.draw(self.renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 898, in draw
    func(*args)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 1997, in draw
    a.draw(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/axis.py", line 1045, in draw
    tick.draw(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/axis.py", line 239, in draw
    self.label1.draw(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/text.py", line 591, in draw
    ismath=ismath)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 156, in draw_text
    return self.draw_mathtext(gc, x, y, s, prop, angle)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 145, in draw_mathtext
    x = int(x) + ox
ValueError: cannot convert float NaN to integer

Answer 1

当我把设置绘图窗口那一行的注释去掉时，它就能正常运行了。

import matplotlib.pyplot as plt
import numpy as np
import matplotlib.cm as cm

bazbins = np.linspace(0, 2*np.pi, 360)
fbins = np.logspace(np.log10(0.05), np.log10(0.5), 101)
theta, r = np.meshgrid(bazbins, fbins) 

# Set up plot window
fig, ax = plt.subplots(figsize=(12,9), subplot_kw=dict(projection='polar'))

# polar
ax.set_theta_zero_location('N')
ax.set_theta_direction(-1)
ax.set_rscale('log')

plt.gca().invert_yaxis()
ax.contourf(theta, r, r)
ax.set_ylim((0.0, 0.5))
plt.show() 

Answer 2

更新内容：使用 matplotlib v3.3.4

• 更新 def scatter_logpolar_mpl 函数，改用 'symlog'，因为使用 'log' 会导致出现 posx and posy should be finite values 的错误，并且 'log-polar matplotlib' 会显示为空白图。

# updated function with symlog
def scatter_logpolar_mpl(ax, theta, r):
    ax.scatter(theta, r)
    ax.set_rlim(0)
    ax.set_rscale('symlog')
    ax.set_title('log-polar matplotlib')

# use other unchanged original functions

# setup the plot
r = np.arange(0, 3.0, 0.01) + 0.001

theta = 2 * np.pi * r

ax = plt.subplots(1, 3, subplot_kw=dict(polar=True), figsize=(12, 7))[1].flatten()
scatter_polar_mpl(ax[0], theta, r)
scatter_logpolar_mpl(ax[1], theta, r)
scatter_logpolar(ax[2], theta, r)

plt.tight_layout()
plt.show()

原始回答

当前的 matplotlib 和对数极坐标图还有更多问题。

比如，试着在 matplotlib 的极坐标图示例 中给半径加一个小值，然后使用 set_rlim(0) 和 set_rscale('log') 来绘制（正如这里评论中提到的）。所有小于 0.1 的值会被特殊处理。这会影响 r 轴上的刻度（注意完全错位的 10e-2 和 10e-3）以及绘制的数据：

这种行为似乎没有文档说明。我最后手动进行了对数变换（上面系列中的第三个图）。对于其他看到这个讨论的人，这里是我的代码：

import numpy as np
import matplotlib.pyplot as plt

def scatter_polar_mpl(ax, theta, r):
    ax.scatter(theta, r)
    ax.set_rlim(0)
    ax.set_title('polar matplotlib')
    
def scatter_logpolar_mpl(ax, theta, r):
    ax.scatter(theta, r)
    ax.set_rlim(0)
    ax.set_rscale('log')
    ax.set_title('log-polar matplotlib')
    
def scatter_logpolar(ax, theta, r_, bullseye=0.3, **kwargs):
    min10 = np.log10(np.min(r_))
    max10 = np.log10(np.max(r_))
    r = np.log10(r_) - min10 + bullseye
    ax.scatter(theta, r, **kwargs)
    l = np.arange(np.floor(min10), max10)
    ax.set_rticks(l - min10 + bullseye) 
    ax.set_yticklabels(["1e%d" % x for x in l])
    ax.set_rlim(0, max10 - min10 + bullseye)
    ax.set_title('log-polar manual')
    return ax
    
r = np.arange(0, 3.0, 0.01) + 0.001

theta = 2 * np.pi * r

ax = plt.subplots(1, 3, subplot_kw=dict(polar=True))[1].flatten()
scatter_polar_mpl(ax[0], theta, r)
scatter_logpolar_mpl(ax[1], theta, r)
scatter_logpolar(ax[2], theta, r)

plt.show()

Answer 3

这看起来是matplotlib中的一个错误。在调用set_rscale（或set_yscale）之前，你不应该需要先调用set_rlim（或set_ylim）这个方法。此外，你必须在调用set_rlim或set_ylim时，把下限设置为0或0.0。如果下限设置为其他值，也会导致程序崩溃。这个问题在其他后端也会出现（我已经确认在gtkagg和pdf后端也有这个问题）。

我为这个问题提交了一个错误报告，你可以在这里找到。如果这个问题影响到了你，请去错误报告页面留言，让matplotlib的开发者知道这个问题对用户来说很重要。