How can I shade confidence intervals on a loglog plot?

I am trying to plot frequency spectra with 95% confidence intervals (upper and lower bounds). I am using pwelch to calculate power spectra then plotting using loglog. But so far I can only figure out how to plot the confidence intervals at line. I would like to shade the intervals instead. I think I could do so by shading between the power line and the confidence interval lines. Any ideas? This is my current code and I have attached a figure.

Thank you!

figure

clear PxxU F PxxC; [PxxU,F,PxxC] = pwelch(detrend(fixgaps(PK1.zavg_pE(j,pkperiod))),[],[],nfft,288);

h=loglog(F,PxxU,F,PxxC,'--','color','k');

%h=loglog(F,PxxU);

hold on

grid on

set(gca,'fontsize',20)

xlabel('f (cpd)','fontsize',20)

ylabel('S_{along} (days)','fontsize',20)

title('Alongshore Velocity');

set(gca,'fontsize',16)

xlim([.1 10])

% legend('PK','PO','location','southwest')

legend('PK','PK upper','PK lower','location','southwest')

Answer 1

[Hiro]

x = -10:0.1:10; % between intersections
yy1 =x.^2; %lower function
yy2 = -(x.^2-16); % upper function
x = [x,x];        % repeat x values
yy = [yy1,yy2];   % vector of upper & lower boundaries
fill(x,yy,'g')    % fill area defined by x & yy in green

Hope this helps you out!

[Heidi Hirsh]

Thank you Hiro! I am wondering how I can apply this to a loglog plot? I think in my case yy = PxxC (the upper and lower confidence bounds).

I tried fill(loglog(F,PxxC,'g')) and loglog(fill(F,PxxC,'g'))

Neither works. Is there another way to combine the fill function with log axes?

[Hiro]

https://jp.mathworks.com/help/matlab/ref/fill.html

I'm sure you can find a solution by referring to the documentation! Good luck.

Answer 2

[Star Strider]

Try this:

x = linspace(0.1, 10, 100);                                                 % Create Data 
y = sin(2*pi*x/5)+1.5;                                                      % Create Data 
err = [y+0.2; y-0.2];                                                       % Create Data 

figure
plot(x, y)                                                                  % Mean
hold on
patch([x, fliplr(x)], [err(1,:), fliplr(err(2,:))], 'g', 'FaceAlpha',0.5)   % Shaded Confidence Intervals
hold off
set(gca, 'XScale','log', 'YScale','log')

producing:

[Heidi Hirsh]

Thank you Star Strider! I'm getting closer! But something weird is happening for the my yaxis values between ~0.25 and 1.75 where it is shading outside the confidence intervals instead of inside. I'm not sure why.

I incorporated your code like this:

figure

sgtitle('Current Spectra, (August 9 - October 2, NFFT=8192)')

P = .95; %confidence interval

clear nfft; nfft = 8192;

jdo = 1;

j=jdo

hold on

clear PxxU F PxxC; [PxxU,F,PxxC] = pwelch(detrend(fixgaps(PK1.zavg_pE(j,pkperiod))),[],[],nfft,288);

% h=loglog(F,PxxU,F,PxxC,'--','color','k');

loglog(F,PxxU);

patch([F, fliplr(F)], [PxxC(:,1), fliplr(PxxC(:,2))],'g','FaceAlpha',0.5)

grid on

set(gca,'fontsize',20)

xlabel('f (cpd)','fontsize',20)

ylabel('S_{along} (days)','fontsize',20)

set(gca,'fontsize',16)

xlim([.1 10])

[Star Strider]

You can most likely resolve that problem by sorting all your data by the independent variable. It appears to be ‘wrapping’, so it returns to fhe first value at the end of the data.