在matplotlib轴上设置科学限制后调整指数文本

Question

目前,如果我将matplotlib y轴刻度标签设置为科学模式,它会在表格的y轴顶部给出一个指数 1e-5

我想将其调整为读取,r'$\mathregular{10^{-5}}$'以便打印出来.

这是我的示例代码:

# Create a figure and axis
fig, ax = plt.subplots()

# Plot 100 random points 
# the y values of which are very small
ax.scatter(np.random.rand(100), np.random.rand(100)/100000.0)

# Set the y limits appropriately
ax.set_ylim(0, 1/100000.0)

# Change the y ticklabel format to scientific format
ax.ticklabel_format(axis='y', style='sci', scilimits=(-2, 2))

# Get the offset value
offset = ax.yaxis.get_offset_text()

# Print it out
print '1st offset printout: {}'.format(offset)

# Run plt.tight_layout()
plt.tight_layout()

# Print out offset again - you can see the value now!
print '2nd offset printout: {}'.format(offset)

# Change it to latex format
offset.set_text(r'$\mathregular{10^{-5}}$')

# Print it out
print '3rd offset printout: {}'.format(offset)

# Add some text to the middle of the figure just to 
# check that it isn't the latex format that's the problem
ax.text(0.5, 0.5/100000.0, r'$\mathregular{10^{-2}}$')

# And show the figure
plt.show()

我的输出如下:

1st offset printout: Text(0,0.5,u'')
2nd offset printout: Text(0,636.933,u'1e\u22125')
3rd offset printout: Text(0,636.933,u'$\\mathregular{10^{-5}}$')

您可以在此处找到代码和输出图.

有两个奇怪的地方:一个是我不能覆盖y轴顶部的1e-5(这是目标),另一个是我必须运行plt.tight_layout()才能看到unicode值为偏移.

谁能告诉我哪里出错了？

谢谢

编辑:原始问题没有说清楚我想自动检测当前计算的指数ticklabel_format.因此,不应将设置字符串传递给偏移文本,而应自动检测该值并相应地调整乳胶字符串.

Answer 1

以@edsmith 的回答为基础，一种可能的解决方法是获取偏移文本，将其转换为乳胶字符串，关闭偏移并在轴的顶部添加该字符串。

def format_exponent(ax, axis='y'):

    # Change the ticklabel format to scientific format
    ax.ticklabel_format(axis=axis, style='sci', scilimits=(-2, 2))

    # Get the appropriate axis
    if axis == 'y':
        ax_axis = ax.yaxis
        x_pos = 0.0
        y_pos = 1.0
        horizontalalignment='left'
        verticalalignment='bottom'
    else:
        ax_axis = ax.xaxis
        x_pos = 1.0
        y_pos = -0.05
        horizontalalignment='right'
        verticalalignment='top'

    # Run plt.tight_layout() because otherwise the offset text doesn't update
    plt.tight_layout()
    ##### THIS IS A BUG 
    ##### Well, at least it's sub-optimal because you might not
    ##### want to use tight_layout(). If anyone has a better way of 
    ##### ensuring the offset text is updated appropriately
    ##### please comment!

    # Get the offset value
    offset = ax_axis.get_offset_text().get_text()

    if len(offset) > 0:
        # Get that exponent value and change it into latex format
        minus_sign = u'\u2212'
        expo = np.float(offset.replace(minus_sign, '-').split('e')[-1])
        offset_text = r'x$\mathregular{10^{%d}}$' %expo

        # Turn off the offset text that's calculated automatically
        ax_axis.offsetText.set_visible(False)

        # Add in a text box at the top of the y axis
        ax.text(x_pos, y_pos, offset_text, transform=ax.transAxes,
               horizontalalignment=horizontalalignment,
               verticalalignment=verticalalignment)
    return ax

请注意，您应该能够通过调用来使用偏移文本的位置，pos = ax_axis.get_offset_text().get_position()但这些值不是以轴为单位（它们可能是像素单位 - 感谢 @EdSmith - 因此不是很有帮助）。因此，我只是根据我们正在查看的任何轴设置x_pos和y_pos值。

我还编写了一个小函数来自动检测适当的 x 和 y 限制（尽管我知道 matplotlib 有很多奇特的方法来做到这一点）。

def get_min_max(x, pad=0.05):
    '''
    Find min and max values such that
    all the data lies within 90% of
    of the axis range
    '''
    r = np.max(x) - np.min(x)
    x_min = np.min(x) - pad * r
    x_max = np.max(x) + pad * r
    return x_min, x_max

因此，从问题中更新我的示例（稍作更改以使两个轴都需要指数）：

import matplotlib.pylab as plt
import numpy as np

# Create a figure and axis
fig, ax = plt.subplots()

# Plot 100 random points that are very small
x = np.random.rand(100)/100000.0
y = np.random.rand(100)/100000.0
ax.scatter(x, y)

# Set the x and y limits
x_min, x_max = get_min_max(x)
ax.set_xlim(x_min, x_max)
y_min, y_max = get_min_max(y)    
ax.set_ylim(y_min, y_max)

# Format the exponents nicely
ax = format_exponent(ax, axis='x')
ax = format_exponent(ax, axis='y')

# And show the figure
plt.show()

与显示代码的输出IPython的笔记本电脑涉及的插头可在这里。

我希望这有帮助！

Answer 2

似乎plt.ticklabel_format不能正常工作。但是，如果您自己定义 ScalarFormatter并将科学记数法的限制设置为格式化程序，则可以像这样以 mathtext 格式自动获取偏移量：

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

x = np.linspace(3,5)
y = np.sin(np.linspace(0,6*np.pi))*1e5

plt.plot(x,y)

mf = matplotlib.ticker.ScalarFormatter(useMathText=True)
mf.set_powerlimits((-2,2))
plt.gca().yaxis.set_major_formatter(mf)

plt.show()