Non-prime difficult to divide numbers

import matplotlib.pyplot as plt
import numpy as np

N = 200

def is_prime(n):
    i=2
    if n<i:
        return False
    for x in range(i, n):
        if n%x==0:
            return False
    return True

prime_numbers = []
for i in range(1,N):
    prime = is_prime_alternative(i)
    if prime:
        prime_numbers.append(i)


def lowest_divisor(x):
    if x < 2:
        print("Error. Number too small.")
    if x in [2,3,4,5]:
        return [2,3,2,5][x-2]
    lowest_divisor = 1
    for i in range(2,x//2):
        if x%i == 0:
            lowest_divisor = i
            return lowest_divisor
        if i == x//2-1:
            return x


ratiolist = []
for i in range(2,N):
    ratio = float(i)/float(lowest_divisor(i))
    ratiolist.append(ratio)

x = np.arange(2,N,1)

# x and y ("ratiolist") without the prime numbers:

x_filtered = [value for value in x if not is_prime(value)]
ratiolist_filtered = []
for i in range(2,N):
    if not is_prime(i):
        ratio = float(i)/float(lowest_divisor(i))
        ratiolist_filtered.append(ratio)

lowest_divisor_list = []
for i in range(2,N):
    if not is_prime(i):
        lowest_divisor_list.append(lowest_divisor(i))

# plot
fig0, ax0 = plt.subplots(figsize=(15,5))
ax0.scatter(x_filtered,lowest_divisor_list)
ax0.set_title('The lowest divisor.')
plt.ylabel("lowest divisor for x", fontsize=12)
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(15,5))
ax1.scatter(x, ratiolist)
ax1.set_title('With prime numbers')
ax1.set_ylabel("ratio of x to its lowest divisor", fontsize=12)
ax2.scatter(x_filtered, ratiolist_filtered)
ax2.set_title('Without prime numbers')
ax2.set_ylabel("ratio of x to its lowest divisor", fontsize=12)




# ignore this code snippet, it's from another project:

def intersection(l1, l2):
  l3 = [value for value in l1 if value in l2]
  return l3
def overhang(l1, l2):
  l3 = [value for value in l1 if value not in intersection(l1,l2)]
  return l3