My Fourier Series plotter isn't outputting correct y-values

This code outputs the power spectrum of the calculated Fourier Series. I'd like to output the y values of the Fourier Series for each x but the values are not correct (supposed to be a smooth Fourier Series curve).

Any ideas? Any help would be greatly appreciated.

%%
%Coefficients (with 95% confidence bounds):
Ts = 1800; %Time series interval (in seconds)
x = 0:Ts:(length(WavePower.Data)*1800); %Interval (Length - 89136)
a0 =       28.46;
a1 =       21.17;
b1 =       2.302;
a2 =     -0.3387;
b2 =         2.8;
a3 =       -4.94;
b3 =       3.124;
w =     0.000353;
% Goodness of fit:
%   SSE: 1.862e+08
%   R-square: 0.106
%   Adjusted R-square: 0.106
%   RMSE: 45.71
%General model Fourier3:
yfit =  a0 + a1*cos(x*w) + b1*sin(x*w) + a2*cos(2*x*w) + b2*sin(2*x*w) + a3*cos(3*x*w) + b3*sin(3*x*w);
% Compute the two-sided spectrum P2. Then compute the single-sided spectrum P1 based...
T = 60*30;           %Sampling period
Fs = 1/T;            %Sampling frequency
L = length(WavePower.Data); %Length of singal (89136) - no inputs
t = (0:L-1)*T;              %Time vector
Y = fft(yfit);              %Where yfit is your Fourier Series
%Define the frequency domain f and plot the single-sided amplitude spectrum P1. The a.........
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;         %Frequency domain
plot(f,P1)
title('Single-Sided Amplitude Spectrum of X(t)')
xlabel('f (Cycles/30 minutes)')
ylabel('|P1(f)|')

Answer 1

[VBBV]

plot(yfit(1:Ts:end))  % plot your fourierseries 

It seems you have already computed the fourier series, To visualize the smooth curve that you're referring plot only yfit