figure设置坐标轴

import matplotlib.pyplot as plt
import numpy as np

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

plt.figure(num=3,figsize=(8,5))
plt.plot(x,y2)
plt.plot(x,y1,color="red",linewidth=1.0,linestyle="--")

plt.xlim((-1,2))
plt.ylim((-2,3))
plt.xlabel("I am x")
plt.ylabel("I am y")

new_ticks=np.linspace(-1,2,5)
print(new_ticks)
plt.xticks(new_ticks)
plt.yticks([-2,-1.8,-1,1.22,3,],
        [r'$really\ bad$',r'$bad\ \alpha$',r'$normal$',r'$good$','$really\ good$'] 

  #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))
  ax.spines['left'].set_position(('data',0))

  plt.show()