Matplotlib loglog的错误标记/标签（双轴）

Question

我正在使用matplotlib创建loglog图。从下图中可以看出，默认的刻度选择很差（最多）;正确的y轴甚至根本没有任何东西（它在线性等效物中），而且两个x轴只有一个。

有没有办法通过标签获得合理数量的刻度，没有为每个绘图手动指定它们？

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编辑：确切的代码太长了，但这是一个问题的简短例子：

x = linspace(4, 18, 20)
y = 1 / (x ** 4)
fig = figure()
ax = fig.add_axes([.1, .1, .8, .8])
ax.loglog(x, y)
ax.set_xlim([4, 18])
ax2 = ax.twiny()
ax2.set_xlim([4 / 3., 18 / 3.])
ax2.set_xscale('log')
show()

Answer 1

我一直在与你所展示的东西作斗争（轴范围内只有一个主要的刻度线）。 matplotlib tick格式化程序都没有让我满意，所以我使用matplotlib.ticker.FuncFormatter来实现我想要的。我没有用双轴进行测试，但我的感觉是它无论如何都应该工作。

import matplotlib.pyplot as plt
from matplotlib import ticker
import numpy as np

#@Mark: thanks for the suggestion :D
mi, ma, conv = 4, 8, 1./3.
x = np.linspace(mi, ma, 20)
y = 1 / (x ** 4)

fig, ax = plt.subplots()

ax.plot(x, y)  # plot the lines
ax.set_xscale('log') #convert to log
ax.set_yscale('log')

ax.set_xlim([0.2, 1.8])  #large enough, but should show only 1 tick

def ticks_format(value, index):
    """
    This function decompose value in base*10^{exp} and return a latex string.
    If 0<=value<99: return the value as it is.
    if 0.1<value<0: returns as it is rounded to the first decimal
    otherwise returns $base*10^{exp}$
    I've designed the function to be use with values for which the decomposition
    returns integers
    """
    exp = np.floor(np.log10(value))
    base = value/10**exp
    if exp == 0 or exp == 1:
        return '${0:d}$'.format(int(value))
    if exp == -1:
        return '${0:.1f}$'.format(value)
    else:
        return '${0:d}\\times10^{{{1:d}}}$'.format(int(base), int(exp))

# here specify which minor ticks per decate you want
# likely all of them give you a too crowed axis
subs = [1., 3., 6.]
# set the minor locators
ax.xaxis.set_minor_locator(ticker.LogLocator(subs=subs))
ax.yaxis.set_minor_locator(ticker.LogLocator(subs=subs))
# remove the tick labels for the major ticks: 
# if not done they will be printed with the custom ones (you don't want it)
# plus you want to remove them to avoid font missmatch: the above function 
# returns latex string, and I don't know how matplotlib does exponents in labels
ax.xaxis.set_major_formatter(ticker.NullFormatter())
ax.yaxis.set_major_formatter(ticker.NullFormatter())
# set the desired minor tick labels using the above function
ax.xaxis.set_minor_formatter(ticker.FuncFormatter(ticks_format))
ax.yaxis.set_minor_formatter(ticker.FuncFormatter(ticks_format))

我得到的数字是以下：

当然你可以为x和y轴设置不同的次要定位器，你可以将ticks_format到最后的所有东西都包装成一个接受轴实例ax和subs的函数或者subsx和subsy作为输入参数。

我希望这可以帮助你

Answer 2

最后，这是我能在其他答案的帮助下得出的最好的结果，而且是这样的：

在左侧，x和y仅在一个数量级的一部分上变化，标签运行得相当好。在左侧，x在1到2个数量级之间变化。它工作正常，但方法达到了它的极限。 y值变化很多个数量级，标准标签会自动使用。

from matplotlib import ticker
from numpy import linspace, logspace, log10, floor
from warnings import warn

def round_to_n(x, n):
    ''' http://stackoverflow.com/questions/3410976/how-to-round-a-number-to-significant-figures-in-python '''
    return round(x, -int(floor(log10(abs(x)))) + (n - 1))

def ticks_log_format(value, index):
    ''' http://stackoverflow.com/questions/19239297/matplotlib-bad-ticks-labels-for-loglog-twin-axis '''
    pwr = floor(log10(value))
    base = value / (10 ** pwr)
    if pwr == 0 or pwr == 1:
        return '${0:d}$'.format(int(value))
    if -3 <= pwr < 0:
        return '${0:.3g}$'.format(value)
    if 0 < pwr <= 3:
        return '${0:d}$'.format(int(value))
    else:
        return '${0:d}\\times10^{{{1:d}}}$'.format(int(base), int(pwr))

def calc_ticks(domain, tick_count, equidistant):
    if equidistant:
        ticks = logspace(log10(domain[0]), log10(domain[1]), num = tick_count, base = 10)
    else:
        ticks = linspace(domain[0], domain[1], num = tick_count)
    for n in range(1, 6):
        if len(set(round_to_n(tick, n) for tick in ticks)) == tick_count:
            break    
    return list(round_to_n(tick, n) for tick in ticks)

''' small domain log ticks '''
def sdlt_x(ax, domain, tick_count = 4, equidistant = True):
    ''' http://stackoverflow.com/questions/3410976/how-to-round-a-number-to-significant-figures-in-python '''
    if min(domain) <= 0:
        warn('domain %g-%g contains values lower than 0' % (domain[0], domain[1]))
        domain = [max(value, 0.) for value in domain]
    ax.set_xscale('log')
    ax.set_xlim(domain)
    ax.xaxis.set_major_formatter(ticker.FuncFormatter(ticks_log_format))
    if log10(max(domain) / min(domain)) > 1.7:
        return
    ticks = calc_ticks(domain, tick_count = tick_count, equidistant = equidistant)
    ax.set_xticks(ticks)

''' any way to prevent this code duplication? '''
def sdlt_y(ax, domain, tick_count = 5, equidistant = True):
    ''' http://stackoverflow.com/questions/3410976/how-to-round-a-number-to-significant-figures-in-python '''
    if min(domain) <= 0:
        warn('domain %g-%g contains values lower than 0' % (domain[0], domain[1]))
        domain = [max(value, 1e-8) for value in domain]
    ax.set_yscale('log')
    ax.set_ylim(domain)
    ax.yaxis.set_major_formatter(ticker.FuncFormatter(ticks_log_format))
    if log10(max(domain) / min(domain)) > 1.7:
        return
    ticks = calc_ticks(domain, tick_count = tick_count, equidistant = equidistant)
    ax.set_yticks(ticks)

''' demo '''
fig, (ax1, ax2,) = plt.subplots(1, 2)
for mi, ma, ax in ((100, 130, ax1,), (10, 400, ax2,), ):
    x = np.linspace(mi, ma, 50)
    y = 1 / ((x + random(50) * 0.1 * (ma - mi)) ** 4)
    ax.scatter(x, y)
    sdlt_x(ax, (mi, ma, ))
    sdlt_y(ax, (min(y), max(y), ))
show()

编辑：更新了一个选项，使标签等距（因此值是对数的，但可见位置是等距的）。