plot cdf and calculate 90th percentile value

hello all ,

i have a variable z_cdf attached;

prctile(z_cdf,90) % answer is 30.9361

now using z_cdf i am trying to plot cdf.

[f,x] = ecdf(z_cdf);
plot(x,f,'HandleVisibility','off','color',[1 0 0],'LineWidth',2);

now in the plot i need to show the line representing the 90th percentile value as shown in the below figure. i.e. the line corresponding to cdf / y-axis= 0.9;

please help me with the line plotting option and also

according to plot x-axis value corresponding to y-axis value 0.9 is 30.7553. why is this x value not equal to prctile(z_cdf,90).

help me with similar cdf plot options where prctile(z_cdf,90) matches (prctile(z_cdf,90),0.9) in plot.

Answer 1

[Star Strider]

To plot the dashed lines at the appropriate percentiles:

pctl = [50 90];
pctlv = prctile(z_cdf,[50 90]); % answer is 30.9361

[f,x] = ecdf(z_cdf);

figure
plot(x,f,'color',[1 0 0],'LineWidth',2)
hold on
for k = 1:numel(pctl)
    plot([1;1]*pctlv(k), [0;1]*pctl(k)/100, '--k')
    plot([0;1]*pctlv(k), [1;1]*pctl(k)/100, '--k')
end
hold off

producing:

.