matplotlib库中gca、annotate应用

import numpy as np
import matplotlib.pyplot as plt
xnum = [-2*np.pi,-np.pi,0,np.pi,2*np.pi]
xlabel = [r'$-2\pi$',r'$-\pi$',r'$0$',r'$\pi$',r'$2\pi$']
ynum = [-1,0,1,2,3]
ylabel = [r'$-1$',r'$0$',r'$1$',r'$2$',r'$3$']
plt.title('waOO',fontsize=20,color='pink')
x = np.arange(-2*np.pi,2*np.pi,0.01)
y = np.sin(3*x)/x
y2 = np.sin(2*x)/x
y3 = np.sin(x)/x
plt.plot(x,y,'b')
plt.plot(x,y2,'r')
plt.plot(x,y3,'g')
plt.xticks(xnum,xlabel)
plt.yticks(ynum,ynum)
plt.annotate(r'$\lim_{x\to 0}\frac{sin(x)}{x}=1$',xy=[0,1],xycoords='data',
            xytext=[30,30],fontsize=16,textcoords='offset points',
             arrowprops=dict(arrowstyle='->',connectionstyle='arc3,rad=.2'))
ax = plt.gca()
ax.spines['right'].set_color('none')