ISA Calculator

import matplotlib.pyplot as plt

goal = 100000
in_years = True

# Help To Buy
def htb(P=1200, r=0.04, c=200):
    m = 1
    balance = []
    months = []
    while P <= goal:
        if m % 12 == 0:
            gains = (P * r)
            P += gains
        if in_years:
            months.append(m / 12.)
        else:
            months.append(m)
        balance.append(P)
        m += 1
        P += c
    return months, balance

# Lifetime
def lft(P=1200, r=0.075, c=4000/12.):
    m = 1
    balance = []
    months = []
    while P <= goal:
        if m % 12 == 0:
            gains = (P * r)
            P += gains
            P += 1000
        if in_years:
            months.append(m / 12.)
        else:
            months.append(m)
        balance.append(P)
        m += 1
        P += c
    return months, balance

# Calculate
htb_m, htb_b = htb()
lft_m, lft_b = lft()

# Plot
plt.plot(htb_m, htb_b)
plt.plot(lft_m, lft_b)
plt.ylim(0, goal+1000)
plt.legend(['HelpToBuy ISA', 'Lifetime ISA'])
plt.ylabel('Balance')
x_label = 'Years' if in_years else 'Months'
plt.xlabel(x_label)
plt.title('Time taken to achieve goal of {}'.format(goal))
plt.grid()
plt.show()