Equally spaced markers in a loglog plot

Hello,

I have the following problem: I am plotting a line in a loglog plot. However since the figure will be printed in black&white I want additional markers to be on the line. Is there a simple way to have the makers equally spaced in the loglog scale. The way I am doing it right now is:

x=linspace(1000,10000,1000);
data=x.^-0.5;
loglog(x,data)
hold on
scatter(x(1:100:end),data(1:100:end));

But obviously this makes the marker not equally spaced. I could make a function for 'data' and than feed it with the right values,like this:

xx=(10*ones(1,11)).^(3:0.1:4);
data_fun=@(a) a.^-0.5;
figure()
loglog(x,data)
hold on
scatter(xx,data_fun(xx));

but I was wondering if there is a simpler way, since that would mean creating a lot of functions (in my case). Thx Vali

Answer 1

[Chad Greene]

x=linspace(1000,10000,1000);
data=x.^-0.5;
loglog(x,data)
hold on
evenMarkers(x,data,11);

where evenMarkers is this function:

function [xmarkers,ymarkers,h] = evenMarkers(x,y,NumMarkers)
  xmarkers = logspace(log10(x(1)),log10(x(end)),NumMarkers);
  ymarkers = interp1(x,y,xmarkers);
  h = plot(xmarkers,ymarkers,'ko'); 
end

[Vali]

seems like a very elegant solution. I will use that.

Thank you Chad!

[Matthew]

Excellent solution Chad, but I am going to suggest a small improvement. If plotting linearly spaced data on a loglog plot, say the results from a fft of a time signal, the distance between points at low frequencies can be quite large. Doing the fit in linear space causes an issue here because a line in linear space is not a line in log space. So I recommend the following adjustment:

function [xmarkers,ymarkers] = evenMarkers(x,y,NumMarkers)
  xmarkers = logspace(log10(x(1)),log10(x(end)),NumMarkers);
  ymarkers = interp1(log10(x),log10(y),log10(xmarkers));
  ymarkers = 10.^(ymarkers);
end

Answer 2

[Chad Greene]

You can use logspace. :)

[Vali]

logspace is a nice function to help creating the variable xx so instead of

xx=(10*ones(1,11)).^(3:0.1:4);
I can do

xx=logspace(3,4,10)
but that still does not solve the problem of plotting the markers, at least I don't understand how.