How can I fix plot problems when allocating memory outside of a loop?

I'm using serial port to collect data. Inside of the loop, I have a vector to store the data points and then plot in a graph, but I need to allocate memory outside of the loop. When I do it, all the data points start from the origin (like the picture below). I'm allocating memory this way:

v2 = zeros(100,1);
v1 = zeros(100,1);
t = zeros(100,1);

How can I fix it?

Thank you.

[Walter Roberson]

Please show the code for storing in the arrays, including the initialization of any index variables.