MATLAB: Color plot above and below y-axis points

MATLABplot

From my simulation file below, I wanted to color the plot in red for sections of the graph that goes above 100, and below -100. I bascially wanted the plot to look like this:
I have also attached the data file.
Here is my code:
%=======================================================================

% This Matlab Program reads in the data from a text file.
%=======================================================================
% Select file
clear;
clc;
[FileName,PathName] = uigetfile('*.txt','Select data file');
fid = fopen( strcat(PathName,FileName) ,'rt' );
% Read the file and store into matrix v
i = 0;
v = [0 0];
while feof(fid) == 0
    buffer = fscanf(fid, '%f', 4);
    buffer = buffer';
    if i == 0;
        v = buffer;
    else
        v = vertcat(v,buffer);
    end
    i = i + 1;
end
% Time Vector in ms
time = v(:,1);
time_ms = v(:,1).*1000;
% Freq Data
Freq = v(:,2);
% + Spec 1

Spec1 = v(:,3);
% + Spec 1
Spec2 = v(:,4);
plot(time_ms, Freq, time_ms, Spec1, 'r', time_ms, Spec2, 'r'); grid on; axis([0 7 -500 500]);
title('Zoomed-In Lock Time Response'); legend('Data 1', '+ Spec', '- Spec');
xlabel('Time (ms)'); ylabel('Frequency Offset (Hz)');
return

Best Answer

This is one way to have the plot appear as you want it to. You will have to experiment to get it exactly the way you want:
x = linspace(0, 8, 800);                    % Create Data

y = 1000*randn(1,800).*exp(-0.7*x);         % Create Data
figure(1)
patch([0 8 8 0], [100 100 500 500], 'r', 'FaceAlpha',0.25)
patch([0 8 8 0], -1*[100 100 500 500], 'r', 'FaceAlpha',0.25)
hold on
axis([0  8    -500  500])
plot(x, y)
hold off
grid on
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