linear trendline equation

trendline.py

def trendline(x, y):
    # y = mx + b, where m = slope and b = offset
    return "f(x) = {0}x + {1}".format(slope(x, y), offset(x, y))
  
def sum_xy(x, y):
    s = float(0)
    for i in range(len(x)):
        s += x[i] * y[i]
    return s

def sum(x):
    s = float(0)
    for i in range(len(x)):
        s += x[i]
    return s

def sum_of_square(x):
    s = float(0)
    for i in range(len(x)):
        s += x[i]**2
    return s

def slope(x, y):
    n = float(len(x))
    return (n * sum_xy(x, y) - (sum(x) * sum(y))) / (n * sum_of_square(x) - sum(x)**2)

def offset(x, y):
    n = float(len(x))
    return (sum(y) - slope(x, y) * sum(x)) / n