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

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.

[Scott Webster]

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/

Answer 1

[Knut]

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

[Stanley Kubrick]

I've actually just resorted to taking screenshots and overlaying it manually like this:

[Star Strider]

@Stanley Kubrick — You should probably post that also as a Comment to your post Semilog plot with background image.