MATLAB Answers


How to overlay a log-plot on top of an image

Asked by Knut
on 26 Mar 2013
Latest activity Commented on by Scott Webster on 19 Feb 2015
Here is the use-case: You have access to an image of a MATLAB semilogx() plot from a paper, a website or something similar. The original data are not easily available. You want to manually approximate the original vector. If one could overlay a local MATLAB semilogx() plot on top of the figure, that would make the tedious job slightly less tedious.

  1 Comment

If I'm understanding what you are trying to do correctly, you might want to considering using alternative tools to get at the data, several of which exist, e.g.

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1 Answer

Answer by Knut
on 26 Mar 2013
Edited by Knut
on 26 Mar 2013
 Accepted Answer

Here is my suggested solution. Nothing ground-breaking, but a bit fiddly for my skills using axes.
%%generate some trivial data
[b1,a1] = butter(2,1200./48e3);
[b2,a2] = butter(2,1200./48e3,'high');
H1 = freqz(b1,a1,512,48e3);
H2 = freqz(b2,a2,512,48e3);
im = imread('ngc6543a.jpg');
%%do the actual plotting
%define plot extent so that image can be aligned with plot
xrng = [20 30e3]./1000;
yrng = [-60 10];
ax1 = gca;
%get rid of pixel ticklabels
set(ax1,'YTickLabel', [], 'XTickLabel', [])
%color=none to make the image visible
ax2 = axes('Position',get(ax1,'Position'),'Color','none','XScale', 'log');
xlabel('Frequency - kHz')
ylabel('power - dB')
title('Frequency response')
line(W, 20*log10(abs([H1 H2])),'Parent',ax2);
legend('lopass', 'hipass')
Inspired by:


I've actually just resorted to taking screenshots and overlaying it manually like this:
@Stanley Kubrick — You should probably post that also as a Comment to your post Semilog plot with background image.

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