Error while plotting zero crossing detector

I am trying to plot zero crossing detector in matlab using interpolation for a sine wave

How to remove this red marker in code?

What am i doing wrong in code?

clc;
clear all;
close all;

% CONSTANTS
fs=2400;
fo=50;
t=0:200;
t2=0.5; 

% Geneeration of sine wave
 xp=sin(2*pi*(fo/fs)*t);
% figure(3)
% plot(t,xp,'b');
% grid on;
% legend('sine signal')

% upward zero crossing detector
UpZC = @(a) find(a(1:end-1) <= 0 & a(2:end) > 0);	
% downward zero crossing detector
DownZC = @(a) find(a(1:end-1) >= 0 & a(2:end) < 0);  
% interploator
ZeroX = @(x0,y0,x1,y1) x0 - (y0.*(x0 - x1))./(y0 - y1); 
ZXi = sort([UpZC(xp),DownZC(xp)]);
% zerocrossing detector with interpolation
ZX = ZeroX(t(ZXi),xp(ZXi),t(ZXi+1),xp(ZXi+1));
if xp(end)==0
    ZX(end+1) = t(end);
end
xx= zeros(1,length(ZX));
xx1= zeros(1,length(ZX));

for i=1:length(ZX)
    if rem(i,2)==1
%         xx(1,i)=ZX(i)+t2;
        xx1(1,i)=ZX(i)-t2;
    else
%         xx(1,i)=ZX(i)-t2;
          xx1(1,i)=ZX(i)+t2;
    end
end

y1= sin(2*pi*(fo/fs)*t2);

% figure(2)
% plot(t,xp, '-b')
% hold on;
% plot(ZX,zeros(1,length(ZX)),'ro')
% grid on;
% legend('Signal', 'Interpolated Zero-Crossing')

figure(1)
plot(t,xp, '-b')
hold on;
% plot(xx,zeros(1,length(ZX))+y1,'r*')
hold on
plot(xx1,zeros(1,length(ZX))-y1,'r*')
hold on
plot(ZX,zeros(1,length(ZX)),'go')
grid on;

Answer 1

[the cyclist]

To remove all the red stars, comment out this line:

plot(xx1,zeros(1,length(ZX))-y1,'r*')

To remove only the first red star, change that line to

plot(xx1(2:end),zeros(1,length(ZX)-1)-y1,'r*')

[Walter Roberson]

In newer versions of MATLAB, you can also control which points the marker gets plotted for using the MarkerIndices option. So in theory you could instead

plot(xx1, zeros(1,length(ZX))-y1, 'r*', 'MarkerIndices', 2:length(xx1))

....which will not make any visible difference when you are only plotting markers.

[Shanmuka]

Thank you!!!.

I am trying to remove to first red marker as signal starting from zero. Thank you again.