Why are all the values in my appended list the same?

When I plot y as a function of t, the values for a do not change. When I print the appended list I see they are all 0.0. Please help! I'm confused because y as a function of x plots fine. I can't include the actual code, but here is a minimum working example.

import numpy as np 
from math import *
from astropy.table import Table 
import matplotlib.pyplot as plt
from random import random

x = 0
y = 0
t = 0
h = 0.0100 
tf = 40 
N=ceil(tf/h)
tnew = t 
x_list = [x]
y_list = [y]
t_list = [t]

for i in range(N):
    #while y >= 0:
        tnew = t + h*i  
        t = tnew
        print(t)

        #First and second derivatives
        # stuff happens (can't share the code)
        x_new = random()
        y_new = random()

        x = x_new
        y = y_new

        """ appends selected data for ability to plot"""
        x_list.append(x)
        y_list.append(y)
        t_list.append(t)
    #break

""" Plot1"""
plt.plot(t_list,y_list)
plt.show()

""" Plot2"""
plt.plot(x_list,y_list)
plt.show()

First plot I just get a vertical line Second plot is the way it should be

First plot:

Second plot