How to find co-ordinates in a signal trace when each co-ordinate increments by a changing amount

Hi, I am using MATLAN R2020a on a MacOS. I am trying to find the x(time), y(time)) and z(time) co-ordinates of an ECG-signal trace in terms of amplitude versus time. With each cycle, the x, y and z values increment by a 'tau' value which changes with each cycle. I have created arrays to store the co-ordinates but cannot figure out how to index from the 'tau' vector to use in getting the co-ordinates with each cycle. Could someone please help with this:

% Array for x co-ordinates
x = zeros(size(attractor_data,1),1);
x(1:end) = attractor_data(1:end,2);

% Array for y co-ordinates
y = zeros(size(attractor_data,1),1);

% Array for z co-ordinates 
z = zeros(size(attractor_data,1),1);

% Filling the arrays
y(1+tau:end) = attractor_data(1:end - tau, 2); 
z(1+2*tau:end) = attractor_data(1:end - 2*tau, 2);

At the moment, the tau value is the same.