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

2013 3 26
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更新时间：2015 2 19
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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.

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Scott Webster
Scott Webster 2015-2-19
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.
http://arohatgi.info/WebPlotDigitizer/

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Knut
Knut 2013-3-26
编辑：Knut 2013-3-26

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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
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:
http://www.mathworks.com/help/matlab/creating_plots/using-multiple-x-and-y-axes.html
http://www.peteryu.ca/tutorials/matlab/plot_over_image_background

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Stanley Kubrick
Stanley Kubrick 2015-2-19
I've actually just resorted to taking screenshots and overlaying it manually like this:
Star Strider
Star Strider 2015-2-19
@Stanley Kubrick — You should probably post that also as a Comment to your post Semilog plot with background image.

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