# Probability Plot (P-P plot): to what corresponds the straight line?

I want to show that my data cannot fit a given distribution (in my case it's the Pareto type II distribution). I did a P-P plot, i.e on the y-axis I plot my sorted experimental data and on the x-axis the inverse of the theoretical cdf. I added my Matlab code below in case my explanation is not clear.

Most of the time, on the P-P plots, I see a straight line and it's said that if the point follow this line you can assume that your data are distributed according to this distribution. But how do you plot this straight line? Is it just y = x? If so, why? If not, what exactly do we have to plot?

%%% Definition of the probability density function, the cumulative density function
%%% and the inverse density function
pdf = @(v,x) x(2)/x(1) * (1 + v./x(1)).^(-x(2) - 1);
cdf = @(v,x) 1- (1+v./x(1)).^-x(2);
invcdf = @(y,x) x(1).*((1-y).^-(1./x(2))-1);

%%% Function to minimize to find the parameters %%%
fobserved = @(x) -mean(log((pdf(islets_vol,x)))); % islets_vol = my experimental data
options = optimset('Algorithm', 'interior-point',...
'MaxIter', 1000, ...
'MaxFunEvals', 1000);

[xhat_obs,~] = fmincon(fobserved,[0.5;0.5],-eye(2),[0;0],[],[],[],[],[],options);

%%% P-P plot %%%
yvals = sort(islets_vol,'ascend'); % islets_vol = my experimental data
xvals = invcdf((1:numel(islets_vol))/(numel(islets_vol)),xhat_obs);
scatter(xvals,yvals);