图例（Legend）演示
在Matplotlib中绘制图例。
在Matplotlib中有很多方法可以创建和自定义图例。 下面我们将展示一些如何操作的示例。
首先，我们将展示如何为特定线条制作图例。
import matplotlib.pyplot as plt
import matplotlib.collections as mcol
from matplotlib.legend_handler import HandlerLineCollection, HandlerTuple
from matplotlib.lines import Line2D
import numpy as np
t1 = np.arange(0.0, 2.0, 0.1)
t2 = np.arange(0.0, 2.0, 0.01)
fig, ax = plt.subplots()
# note that plot returns a list of lines. The "l1, = plot" usage
# extracts the first element of the list into l1 using tuple
# unpacking. So l1 is a Line2D instance, not a sequence of lines
l1, = ax.plot(t2, np.exp(-t2))
l2, l3 = ax.plot(t2, np.sin(2 * np.pi * t2), '--o', t1, np.log(1 + t1), '.')
l4, = ax.plot(t2, np.exp(-t2) * np.sin(2 * np.pi * t2), 's-.')
ax.legend((l2, l4), ('oscillatory', 'damped'), loc='upper right', shadow=True)
ax.set_xlabel('time')
ax.set_ylabel('volts')
ax.set_title('Damped oscillation')
plt.show()
接下来，我们将演示绘制更复杂的标签。
x = np.linspace(0, 1)
fig, (ax0, ax1) = plt.subplots(2, 1)
# Plot the lines y=x**n for n=1..4.
for n in range(1, 5):
ax0.plot(x, x**n, label="n={0}".format(n))
leg = ax0.legend(loc="upper left", bbox_to_anchor=[0, 1],
ncol=2, shadow=True, title="Legend", fancybox=True)
leg.get_title().set_color("red")
# Demonstrate some more complex labels.
ax1.plot(x, x**2, label="multi\nline")
half_pi = np.linspace(0, np.pi / 2)
ax1.plot(np.sin(half_pi), np.cos(half_pi), label=r"$\frac{1}{2}\pi$")
ax1.plot(x, 2**(x**2), label="$2^{x^2}$")
ax1.legend(shadow=True, fancybox=True)
plt.show()
在这里，我们将图例附加到更复杂的图上。
fig, axes = plt.subplots(3, 1, constrained_layout=True)
top_ax, middle_ax, bottom_ax = axes
top_ax.bar([0, 1, 2], [0.2, 0.3, 0.1], width=0.4, label="Bar 1",
align="center")
top_ax.bar([0.5, 1.5, 2.5], [0.3, 0.2, 0.2], color="red", width=0.4,
label="Bar 2", align="center")
top_ax.legend()
middle_ax.errorbar([0, 1, 2], [2, 3, 1], xerr=0.4, fmt="s", label="test 1")
middle_ax.errorbar([0, 1, 2], [3, 2, 4], yerr=0.3, fmt="o", label="test 2")
middle_ax.errorbar([0, 1, 2], [1, 1, 3], xerr=0.4, yerr=0.3, fmt="^",
label="test 3")
middle_ax.legend()
bottom_ax.stem([0.3, 1.5, 2.7], [1, 3.6, 2.7], label="stem test")
bottom_ax.legend()
plt.show()
现在我们将展示带有多个关键图例的图例条目。
fig, (ax1, ax2) = plt.subplots(2, 1, constrained_layout=True)
# First plot: two legend keys for a single entry
p1 = ax1.scatter([1], [5], c='r', marker='s', s=100)
p2 = ax1.scatter([3], [2], c='b', marker='o', s=100)
# `plot` returns a list, but we want the handle - thus the comma on the left
p3, = ax1.plot([1, 5], [4, 4], 'm-d')
# Assign two of the handles to the same legend entry by putting them in a tuple
# and using a generic handler map (which would be used for any additional
# tuples of handles like (p1, p3)).
l = ax1.legend([(p1, p3), p2], ['two keys', 'one key'], scatterpoints=1,
numpoints=1, handler_map={tuple: HandlerTuple(ndivide=None)})
# Second plot: plot two bar charts on top of each other and change the padding
# between the legend keys
x_left = [1, 2, 3]
y_pos = [1, 3, 2]
y_neg = [2, 1, 4]
rneg = ax2.bar(x_left, y_neg, width=0.5, color='w', hatch='///', label='-1')
rpos = ax2.bar(x_left, y_pos, width=0.5, color='k', label='+1')
# Treat each legend entry differently by using specific `HandlerTuple`s
l = ax2.legend([(rpos, rneg), (rneg, rpos)], ['pad!=0', 'pad=0'],
handler_map={(rpos, rneg): HandlerTuple(ndivide=None),
(rneg, rpos): HandlerTuple(ndivide=None, pad=0.)})
plt.show()
最后，还可以编写定义如何对图例进行样式化的自定义对象。
class HandlerDashedLines(HandlerLineCollection):
"""
Custom Handler for LineCollection instances.
"""
def create_artists(self, legend, orig_handle,
xdescent, ydescent, width, height, fontsize, trans):
# figure out how many lines there are
numlines = len(orig_handle.get_segments())
xdata, xdata_marker = self.get_xdata(legend, xdescent, ydescent,
width, height, fontsize)
leglines = []
# divide the vertical space where the lines will go
# into equal parts based on the number of lines
ydata = ((height) / (numlines + 1)) * np.ones(xdata.shape, float)
# for each line, create the line at the proper location
# and set the dash pattern
for i in range(numlines):
legline = Line2D(xdata, ydata * (numlines - i) - ydescent)
self.update_prop(legline, orig_handle, legend)
# set color, dash pattern, and linewidth to that
# of the lines in linecollection
try:
color = orig_handle.get_colors()[i]
except IndexError:
color = orig_handle.get_colors()[0]
try:
dashes = orig_handle.get_dashes()[i]
except IndexError:
dashes = orig_handle.get_dashes()[0]
try:
lw = orig_handle.get_linewidths()[i]
except IndexError:
lw = orig_handle.get_linewidths()[0]
if dashes[0] is not None:
legline.set_dashes(dashes[1])
legline.set_color(color)
legline.set_transform(trans)
legline.set_linewidth(lw)
leglines.append(legline)
return leglines
x = np.linspace(0, 5, 100)
fig, ax = plt.subplots()
colors = plt.rcParams['axes.prop_cycle'].by_key()['color'][:5]
styles = ['solid', 'dashed', 'dashed', 'dashed', 'solid']
lines = []
for i, color, style in zip(range(5), colors, styles):
ax.plot(x, np.sin(x) - .1 * i, c=color, ls=style)
# make proxy artists
# make list of one line -- doesn't matter what the coordinates are
line = [[(0, 0)]]
# set up the proxy artist
lc = mcol.LineCollection(5 * line, linestyles=styles, colors=colors)
# create the legend
ax.legend([lc], ['multi-line'], handler_map={type(lc): HandlerDashedLines()},
handlelength=2.5, handleheight=3)
plt.show()
