How to plot a biexponential fit?

I have got some data array (P2P) and a time array (t_array) plotted under this simple line of code:

plot(t_array, P2P, 'o')

This works perfectly, but now I want to fit a biexponential fit line through this data, and I don't know how to do so.

I have read the instructions given in this link, but it still doesn't make sense to me. I attempted replicating what was written in the link, as follows:

P2P0=P2P(1);  %setting my La0 as the first element in my y-axis data array

ft = fittype(sprintf('%e+A1*(1-exp(-gamma1*t))+A2*(1-exp(-gamma2*t))',P2P0),...
  'independent','t','dependent', 'P2P' );
opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
opts.Display = 'Off';
opts.Lower = [0 0 0 0];%lower bounds (A1, gamma1, A2, gamma2)
opts.StartPoint = [1 1 1 1];
[fitresult, gof] = fit( t_array, P2P, ft, opts ); %substituted La by P2P and t by t_array
plot(fitresult,t_array,P2P, 'o')

I merely substituted the La data by my P2P data, and time by t_array, but this gives me a horizontal fit line that doesn't match the data which exponentialy decreases (similarly, but not quite, to the data shown in this other link).

Could it be that I must change my lowed bounds (pots.Lower) and my start point (opts.StartPoint)?

I don't understand the meaning of these arrays? Why are there 4 elements in these arrays? What can I do to fit an array through my data?

[Image Analyst]

Unfortunately you forgot to read the posting Community Guidelines (link further down on this page) and forgot to attach your data. People are most likely waiting for that. Attach them after reading:

TUTORIAL: How to ask a question (on Answers) and get a fast answer

[Goncalo Costa]

This has now been added. The data wasn't mine and I was hesitatant in sharing until I had permission. Thank you for your help.

[Goncalo Costa]

An answer to this question had already been given at the following link: https://uk.mathworks.com/matlabcentral/answers/122561-help-with-a-fit

I believe this fits what I need .