Python script does not generate chart as expected

Question

My Python algorithm is running perfectly, I just can’t generate the graph! I couldn’t identify the error... What’s wrong?

import matplotlib.pyplot as plt

va=float(input('Digite o valor a ser aplicado: '))
juros=1.05
inflacao=1.02
saque=25000
vaplicacao=[va]
nsaque=0
vsaque=[]

while saque<=vaplicacao[len(vaplicacao)-1]:
    va=(vaplicacao[len(vaplicacao)-1]*juros)-(saque)
    vaplicacao.append(va)
    nsaque=nsaque+1
    saque=saque*inflacao
    vsaque.append(nsaque)

plt.plot(vsaque,vaplicacao,'b-')
plt.title('Saldo x Saque')
plt.grid(True)
plt.xlabel('Saque ao longo dos anos')
plt.ylabel('Saldo ao longo dos anos')
plt.show()

Answer 1

You are initiating the list vaplicacao with a value:

vaplicacao=[va]

While the list vsaque empty start:

vsaque=[]

As, within the loop, you always add a value in each list, the list vaplicacao at all times will have one more element than vsaque; thus, it is impossible to plot on a graph a curve that has n points on the abscissa axis and n+1 points on the ordinate axis. If you start vsaque with a value possibly work:

vsaque=[nsaque]

In fact, to get the last item on the list, just do lista[-1]; doesn’t need lista[len(lista)-1].

import matplotlib.pyplot as plt

va = float(input('Digite o valor a ser aplicado: '))
juros = 1.05
inflacao = 1.02
saque = 25000
vaplicacao = [va]
nsaque = 0
vsaque = [nsaque]

while saque <= vaplicacao[-1]:
    va = vaplicacao[-1]*juros - saque
    vaplicacao.append(va)
    nsaque += 1
    saque = saque*inflacao
    vsaque.append(nsaque)

plt.plot(vsaque, vaplicacao, 'b-')
plt.title('Saldo x Saque')
plt.grid(True)
plt.xlabel('Saque ao longo dos anos')
plt.ylabel('Saldo ao longo dos anos')
plt.show()

Answer 2

In your case, as the serve count is linear and sequential, you do not need to compile a list for the x-axis of the graph. A matplotlib is able to do this automatically, and your program could become more compact, look at this:

import matplotlib.pyplot as plt

va = float(input('Digite o valor a ser aplicado: '))

juros = 1.05
inflacao = 1.02
saque = 25000
vaplicacao = [va]

while saque <= vaplicacao[-1]:
    va = (vaplicacao[-1] * juros) - saque
    saque *= inflacao
    vaplicacao.append(va)

plt.plot( vaplicacao, 'b-')
plt.plot( vaplicacao, 'ro')

plt.grid(True)
plt.title('Saldo x Saque')
plt.xlabel('Saque ao longo dos anos')
plt.ylabel('Saldo ao longo dos anos')

plt.show()

Exit (va=800000):

Exit (va=600000):

Exit (va=400000):