matplotlib（3）-- legend（图例、说明、解释）,annotate(标注)，text（标注）

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-3, 3, 50)
y1 = 2 * x + 1
y2 = x ** 2

plt.figure()
l1, = plt.plot(x, y1, color = "red", linewidth = 5.0, linestyle = '--', label = 'down')    #指定线的颜色, 宽度和类型
l2, = plt.plot(x, y2, label = "up")

#给figure添加legend（图例、说明、解释）
plt.legend(handles = [l1, l2], labels = ["y1 = 2 * x + 1", "y2 = x ** 2"], loc = "best")

plt.show()

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-3, 3, 50)
y1 = 2 * x + 1
X0 =1
Y0 = 2 * X0 + 1

#figure 1
plt.figure()
plt.plot(x, y1)
plt.scatter(X0, Y0, s = 30, color = 'black')     #在数据线上画点
plt.plot([X0, X0], [Y0, 0], color = "black", linewidth = 3.0, linestyle = '--')       #过一点做垂直于X轴的垂线

#坐标轴的移动 gca = “get current axis”
ax = plt.gca()
ax.spines["right"].set_color("none")
ax.spines["top"].set_color("none")
ax.xaxis.set_ticks_position("bottom")
ax.yaxis.set_ticks_position("left")
ax.spines["bottom"].set_position(("data", 0))   #Set the X and Y coordinates of the sprite simultaneously
ax.spines["left"].set_position(("data", 0))


#在figure上做标注的两种方法
#方法一
#关于annotate的相关参数介绍，参考博文 https://blog.csdn.net/leaf_zizi/article/details/82886755
##########################
plt.annotate(r"$2*x_0 + 1 = %s$"%Y0, xy = (X0, Y0), xytext = (+30, -30),
             textcoords = "offset points", fontsize = 16,
             arrowprops = dict(arrowstyle = "->", connectionstyle = "arc3, rad=.2"))
#方法二
#补充：关于下角标的添加 “_+下角标字母”
#关于text的相关参数介绍，参考博文 https://blog.csdn.net/TeFuirnever/article/details/88947248
##########################
plt.text(-3.7, 3, r"$This is y1 = 2 * x + 1 mu sigma_i alpha_t$",
         fontdict={"size":16, "color":"red"})

plt.show()