用python的matplotlib画标

import math
import pylab as pl
import numpy as np
def gd(x,m,s):
    left=1/(math.sqrt(2*math.pi)*s)
    right=math.exp(-math.pow(x-m,2)/(2*math.pow(s,2)))
    return left*right
def showfigure():
    x=np.arange(-4,5,0.1)
    y=[]
    for i in x:
        y.append(gd(i,0,1))
    pl.plot(x,y)  
    pl.xlim(-4.0,5.0)
    pl.ylim(-0.2,0.5)
#
    ax = pl.gca()
    ax.spines['right'].set_color('none')
    ax.spines['top'].set_color('none')
    ax.xaxis.set_ticks_position('bottom')
    ax.spines['bottom'].set_position(('data',0))
    ax.yaxis.set_ticks_position('left')
    ax.spines['left'].set_position(('data',0))
    #add param
    label_f1 = "$\mu=0,\ \sigma=1$"
    pl.text(2.5,0.3,label_f1,fontsize=15,verticalalignment="top",
            horizontalalignment="left")
    label_f2 = r"$f(x)=\frac{1}{\sqrt{2\pi}\sigma}exp(-\frac{(x-\mu)^2}{2\sigma^2})$"
    pl.text(1.5,0.4,label_f2,fontsize=15,verticalalignment="top"
            ,horizontalalignment="left")
    pl.show()