Графический график Python, разные регионы устанавливают цвета по-разному

Question

Когда я запускаю свой код, он возвращает пустой график.Я не могу понять, почему мой код делает это.Мне также нужен график, когда он получается, когда ml <0 меняет цвет на синий.Когда ml> 0 меняет цвет на другой, а ml = 0, получается другой цвет.Я приведу код ниже.Если бы кто-то мог помочь, это было бы здорово, так как я не очень знаком с питоном.

(ax1) = plt.subplots(1,1, figsize=(8,9),
                              sharex=True)

#initial conditions                        
d= 0.1
eps = 0.000001

#defining the equations as functions
#xnew equation
def f(a,b,x,y):
z = 1-b*y
r  = a*x*z*(1-x*z)
if r>eps:
    return r
else:
    return 0

#ynew equation
def g(b,d,x,y):
    r = (1-d)*y*(1+b*x)
    if r>eps:
        return r
    else:
    return 0

#Equation A
def A(a,b,x,y):
    return a*(1-2*x - b*y + 4*b*x*y - 2*b**2*x*y**2)
#Equation B  
def B(b,a,x,y):
    return a*b*x*(2*x - 1 - 2*b*x*y)
 #Equation C   
def C(b,d,x,y):
    return (1-d)*b*y
#Equation D
def D(b,d,x,y):
    return (1-d)*(1+b*x)
#Equation E, which is max lyapunov exponent equation formed from the parameters above  
def e(a,b,x,y,yn):
    return log((A(a,b,x,y)+(B(b,a,x,y)*yn))**2 +(C(a,b,x,y)+ 
    (D(b,d,x,y)*yn))**2 / (1 + yn**2))
#next ydash value calculated after the max lyapunov exponent is calculated
def yz(a,b,x,y,yn):
    return (C(b,d,x,y)+(D(b,d,x,y)*yn))/(A(a,b,x,y)+(B(b,a,x,y)*yn))    
#defining the function
def lyapunov():
    #initial conditions
    d=0.1
    b=0.4
    xold = 0.5
    yold = 0.01
    #looping values of a and b 0<a<5 and 0<b<6
    for i in range(100):
        a = 1+ 4*i/100
        xold = 0.5
        yold = 0.01
        ydash = 1
        yn=1

        for i in range(100):
            b = 1+ 5*i/100
            xold = 0.5
            yold = 0.01  
            ydash = 1
            yn=1
            #looping round x and y values
            for j in range(100):
                xnew=f(a,b,xold,yold)
                ynew=g(b,d,xold,yold)
                ydash =1 
                #the new value of xnew is xold same for y
                xold = xnew
                yold = ynew
                #setting ml =0 as it starts at 0. inital
                ml = 0
                yn=1
                #calculating the two equations and plotting ml against a and
     #b. Also, need to print out the max ml value along with x and y        
     #value in a text file.

                for k in range(100):
                    outfile = open('lyapunov.txt', 'w')
                    #finding sum of max lyapunov exponent then * by 0.5*N, in this case N is 100
                    ml = e(a,b,xold,yold,yn)
                    ml = ml + e(a,b,xold,yold,yn)
                    ml = (ml/100)*2 
                    #ydash equation
                    ydash = yz(a,b,xold,yold,yn)
                    #now setting xold and yold equal to the equations xnew,ynew were so it keeps looping around each time, xold is no longer 0.5. Keeps taking new values. Same with yold.
                    xold=f(a,b,xold,yold)
                    yold=g(b,d,xold,yold)
                    #writing to file

                   outfile.write(str(xold)+'\t'+str(yolf)+'\t'+str(ml)+'\n')
    #plotting a,ml and b,ml            
    ax1.plot(a, ml,',k')
    ax1.plot(b, ml, ',k')
    #closing the file
    outfile.close() 
#setting graph axis             
ax1.set_ylim(0, 5)        
ax1.set_xlim(0, 6) 
#title
ax1.set_title('Lyapunov Exponent')  

lyapunov()