How to create the time-frequency spectrogram of FSK

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Hello...I'm new in this domain and I want to create a spectrogram of a FSK signal and I don't know how...please help me. Here is the code
format long;
% Clear all variables and close all figures
clear all;
close all;
% The number of bits to send - Frame Length
N = 8;
% Generate a random bit stream
bit_stream = round(rand(1,N));
% Enter the two frequencies
% Frequency component for 0 bit
f1 = 3;
% Frequency component for 1 bit
f2 = 5;
% Sampling rate - This will define the resoultion
fs = 100;
% Time for one bit
t = 0: 1/fs : 1;
% This time variable is just for plot
time = [];
FSK_signal = [];
Digital_signal = [];
for ii = 1: 1: length(bit_stream)
% The FSK Signal
FSK_signal = [FSK_signal (bit_stream(ii)==0)*sin(2*pi*f1*t)+...
(bit_stream(ii)==1)*sin(2*pi*f2*t)];
% The Original Digital Signal
Digital_signal = [Digital_signal (bit_stream(ii)==0)*...
zeros(1,length(t)) + (bit_stream(ii)==1)*ones(1,length(t))];
time = [time t];
t = t + 1;
end
% Plot the FSK Signal
subplot(2,1,1);
plot(time,FSK_signal);
xlabel('Time (bit period)');
ylabel('Amplitude');
title('FSK Signal with two Frequencies');
axis([0 time(end) -1.5 1.5]);
grid on;
% Plot the Original Digital Signal
subplot(2,1,2);
plot(time,Digital_signal,'r','LineWidth',2);
xlabel('Time (bit period)');
ylabel('Amplitude');
title('Original Digital Signal');
axis([0 time(end) -0.5 1.5]);
grid on;

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