How do I get a plot of a parabola graph from the mathematical model(trajectory of a particle) given?

Hi.

I am having a bit of a problem getting the graph of this mathematical model. The graph that I am supposed to get is a parabola showing the trajectory of a particle with drag.(I do get a parabola for the trajectory of the particle without drag.) Instead I keep getting half of a parabola.

I have tried changing the parameters but I still can't seem to get the parabola. The v(velocity),theta(launch angle),r(radius of particle) and x0(initial position) can be changed below.

I would really like some help here.

The equation of the trajectory of the particle with drag is:

x(t) = v_∞ t + m/k (v_0 – v_∞ )(1 - e ^(-kt/m)) + x_0.

The codes for the model that I have written is:

clc; close all; clear all;

% Flight trajectory computation

% Initial values

g = 9.81; % gravity, m/s^2

v = 40; % launch velocity, m/s

theta = 55 * pi/180; % launch angle, radians

m = 5.5*10^-7;

u = 1.78*10^-5;

r = 0.001;

k = 6*pi*u*r;

vinf = m*g/k;

x0 = 20;

% Compute and display results

disp('time of flight (s):') % label for time of flight

tg = 2 * v * sin(theta)/g % time to return to ground, s

disp('distance traveled (ft):') % label for distance

xg = v * cos(theta) * tg % distance traveled

% Compute and plot flight trajectory

t = linspace(0,tg,5000);

x = v * cos(theta) * t;

%y = v * sin(theta) * t - g/2 * t.^2; %trajectory of a particle without drag

y = vinf * sin(theta) * t - m/k * (v * sin(theta) - vinf* sin(theta) * (g/2 * t^2))*(1 - exp(-(k*t)/m)) + x0; %trajectory of a particle with drag

plot(x,y) , axis equal, axis([ 0 300 0 100 ])