左右对齐matplotlib分散标记

Question

我正在使用matplotlibscatterplot函数在垂直线上创建句柄的外观，以描绘图形的某些部分。然而，为了使它们看起来正确，我需要能够将散点图标记向左对齐（对于左线/划定线）和/或向右对齐（对于右线/划定线）。

这里有一个例子：

#create the figure
fig = plt.figure(facecolor = '#f3f3f3', figsize = (11.5, 6))
ax = plt. ax = plt.subplot2grid((1, 1), (0,0))

#make some random data
index = pandas.DatetimeIndex(start = '01/01/2000', freq  = 'b', periods = 100)
rand_levels = pandas.DataFrame( numpy.random.randn(100, 4)/252., index = index, columns = ['a', 'b', 'c', 'd'])
rand_levels = 100*numpy.exp(rand_levels.cumsum(axis = 0))
ax.stackplot(rand_levels.index, rand_levels.transpose())

#create the place holder for the vertical lines
d1, d2 = index[25], index[50]

#draw the lines
ymin, ymax = ax.get_ylim()
ax.vlines([index[25], index[50]], ymin = ymin, ymax = ymax, color = '#353535', lw = 2)

#draw the markers
ax.scatter(d1, ymax, clip_on = False, color = '#353535', marker = '>', s = 200, zorder = 3)
ax.scatter(d2, ymax, clip_on = False, color = '#353535', marker = '<', s = 200, zorder = 3)

#reset the limits
ax.set_ylim(ymin, ymax)
ax.set_xlim(rand_levels.index[0], rand_levels.index[-1])
plt.show()

上面的代码几乎给出了我要找的图形，如下所示：

对如何以灵活的方式实现这一目标有任何指导或建议？

注意：实际上，我的dataframe索引是pandas.datetime而不是我为这个简单示例提供的整数。

Answer 1

我喜欢这个问题，对我的第一个回答不满意。特别是，创建特定于图形的对象(mark_align_*)来对齐标记似乎是不必要的麻烦。我最终找到了通过verts指定标记的功能（2元素浮点列表或Nx2数组，用于指定标记顶点相对于(0,0)处的目标绘图点）。为了利用这个功能,我编写了这个函数,

from matplotlib import markers
from matplotlib.path import Path

def align_marker(marker, halign='center', valign='middle',):
    """
    create markers with specified alignment.

    Parameters
    ----------

    marker : a valid marker specification.
      See mpl.markers

    halign : string, float {'left', 'center', 'right'}
      Specifies the horizontal alignment of the marker. *float* values
      specify the alignment in units of the markersize/2 (0 is 'center',
      -1 is 'right', 1 is 'left').

    valign : string, float {'top', 'middle', 'bottom'}
      Specifies the vertical alignment of the marker. *float* values
      specify the alignment in units of the markersize/2 (0 is 'middle',
      -1 is 'top', 1 is 'bottom').

    Returns
    -------

    marker_array : numpy.ndarray
      A Nx2 array that specifies the marker path relative to the
      plot target point at (0, 0).

    Notes
    -----
    The mark_array can be passed directly to ax.plot and ax.scatter, e.g.::

        ax.plot(1, 1, marker=align_marker('>', 'left'))

    """

    if isinstance(halign, (str, unicode)):
        halign = {'right': -1.,
                  'middle': 0.,
                  'center': 0.,
                  'left': 1.,
                  }[halign]

    if isinstance(valign, (str, unicode)):
        valign = {'top': -1.,
                  'middle': 0.,
                  'center': 0.,
                  'bottom': 1.,
                  }[valign]

    # Define the base marker
    bm = markers.MarkerStyle(marker)

    # Get the marker path and apply the marker transform to get the
    # actual marker vertices (they should all be in a unit-square
    # centered at (0, 0))
    m_arr = bm.get_path().transformed(bm.get_transform()).vertices

    # Shift the marker vertices for the specified alignment.
    m_arr[:, 0] += halign / 2
    m_arr[:, 1] += valign / 2

    return Path(m_arr, bm.get_path().codes)

使用此函数,可以将所需的标记绘制为,

ax.plot(d1, 1, marker=align_marker('>', halign='left'), ms=20,
        clip_on=False, color='k', transform=ax.get_xaxis_transform())
ax.plot(d2, 1, marker=align_marker('<', halign='right'), ms=20,
        clip_on=False, color='k', transform=ax.get_xaxis_transform())

或者使用ax.scatter,

ax.scatter(d1, 1, 200, marker=align_marker('>', halign='left'),
           clip_on=False, color='k', transform=ax.get_xaxis_transform())
ax.scatter(d2, 1, 200, marker=align_marker('<', halign='right'),
           clip_on=False, color='k', transform=ax.get_xaxis_transform())

干杯！