You are getting two plots, but your T_a abd T_b are exactly equal. Thats why they are lying one above the other.
clear all;
close all;
clc;
% Fin Configuration: A
T_s = 500; % [K]
T_inf = 350; % [K]
L = 0.2; % [m]
ST_k = 14.9; % [W/m*K]
U_inf = 10; % [m/s]
D = 0.002; % [m]
nu = 29.4*10^-6; % [m^2/s]
Pr = 0.688; % no units
Re = (U_inf*D)/(nu); % no units
h = ((((0.664*(Re)^(1/2))*((Pr)^(1/3)))*(ST_k))/(L)); % [W/m^2*K]
P = pi*D; % [m]
A_a = pi*D*L; % [m^2]
m = sqrt((h*P)/(ST_k*A_a)); % [1/m]
x = linspace(0,0.15); % [0 to 15cm] or [0 to 0.15m]
Q_a = sqrt(h*P*ST_k*A_a)*(T_s - T_inf)*tanh(m*L); % [W] or [J/s]
T_a = 6.3763*cosh(m*(L-x))+350; % [K]
% Fin Configuration: B
T_s = 500; % [K]
T_inf = 350; % [K]
L = 0.2; % [m]
ST_k = 14.9; % [W/m*K]
U_inf = 10; % [m/s]
D_b = sqrt(0.25*pi*(D^2)); % [m]
nu = 29.4*10^-6; % [m^2/s]
Pr = 0.688; % no units
Re_b = (U_inf*D_b)/(nu); % no units
h_b = ((((0.664*(Re_b)^(1/2))*((Pr)^(1/3)))*(ST_k))/(L)); % [W/m^2*K]
P_b = 4*D; % [m]
A_b = 4*D*L; % [m^2]
m_b = sqrt((h*P)/(ST_k*A_b)); % [1/m]
x = linspace(0,0.15); % [0 to 15cm] or [0 to 0.15m]
Q_b = sqrt(h*P*ST_k*A_b)*(T_s - T_inf)*tanh(m*L); % [W] or [J/s]
T_b = 6.3763*cosh(m*(L-x))+350; % [K]
isequal(T_a,T_b)
figure('Name','Fin A Temp. v. Length')
plot(x,T_a,'r',x,T_b,'.b');
hold on;
set(0,'DefaultAxesFontName','Times New Roman');
title('Temperature v. Distance');
xlabel('Distance (m)','FontWeight','bold','FontSize',10);
ylabel('Temperature (K)','FontWeight','bold','FontSize',10);
grid on;
