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

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Knut on 26 Mar 2013
Commented: 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

Scott Webster on 19 Feb 2015
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.

Knut on 26 Mar 2013
Edited: Knut on 26 Mar 2013
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);
%%do the actual plotting
figure
%define plot extent so that image can be aligned with plot
xrng = [20 30e3]./1000;
yrng = [-60 10];
image(xrng,yrng,im);
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');
ylim(yrng)
xlim(xrng)
xlabel('Frequency - kHz')
ylabel('power - dB')
title('Frequency response')
line(W, 20*log10(abs([H1 H2])),'Parent',ax2);
legend('lopass', 'hipass')
Inspired by: