为什么填充部分少了一块，我该如何解决？

Question

我要在图中[a,b]区间填充颜色，本来想用多边形的方法，但觉得直接填充更直接点，我是用的是fill_betweenx的方法，为设么只有y>=0部分被填充了，下面的部分没有被填充？
代码如下：

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches

#构造数据
x = np.linspace(-2,3,100)
y = -x**2+4


fig,ax = plt.subplots()
line = ax.plot(x,y,lw=2,c='r')
ax.set_xlim(-2,2)
ax.set_ylim(-4,6)

a = -1
b = 1
ax.set_xticks([a,b])
ax.set_xticklabels(['a','b'])
#ax.set_yticks([])

fig.text(0.9,0.05,'x')
fig.text(0.1,0.9,'y')

ax.text(0,-2,r'$\int_{a}^{b}-x^2+4$dx',size=20,family='fantasy',ha='center')


ax.fill_between(x,y,where=(a<=x)&(x<=b))![图片描述][1]

plt.show()

生成效果如下

Answer 1

ax.set_ylim(0,6)
ax.text(0,-1,r'$\int_{a}^{b}-x^2+4$dx',size=20,family='fantasy',ha='center')

ax.set_xlim(-4,4)
ax.set_ylim(-4,5)

a = -2
b = 4

min(x[a<=x]) = -2, min(x[x<=b]) = 3

ax.fill_between(x,y,where=(a<=x)&(x<=b))