How to change axis of graph and interpolate data

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Vaultec 2014-6-26

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https://ww2.mathworks.cn/matlabcentral/answers/137775-how-to-change-axis-of-graph-and-interpolate-data

评论： Star Strider 2014-10-8

采纳的回答： Star Strider

Figure 3#.fig

Sorry new to matlab if this seems like a basic question

Currently I have a plot that I have attached.I want to find the intersections between the plot and the line by interpolating the data. What functions/operations do i need to use on matlab

Thanks in advance

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Star Strider 2014-6-26

It would help if you attached your plot.

The best way to do this is to save it and attach it as a ‘.fig’ file, since that contains your data as well.

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Star Strider 2014-6-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/137775-how-to-change-axis-of-graph-and-interpolate-data#answer_142615

在 MATLAB Online 中打开

This works:

% GET INFORMATON FROM FIGURE: 
openfig('Figure 3#.fig');
h1c = get(gca, 'Children');
Xdc = get(h1c, 'XData');
Xd = cell2mat(Xdc);
Ydc = get(h1c, 'YData');
Yd = cell2mat(Ydc);
% CALCULATIONS: 
Ydn = diff(Yd, [], 1);              % Subtract line from curve to create zero-crossings
Zx = circshift(Ydn, [0 1]) .* Ydn;  % Use circshift to detect them
Zxi = find(Zx < 0);                 % Their indices
for k1 = 1:length(Zxi)              % Use interp1(Y,X,0) to get line intercepts as Xzx
    Xzx(k1) = interp1([Ydn(Zxi(k1)-1) Ydn(Zxi(k1))], [Xd(1,Zxi(k1)-1) Xd(1,Zxi(k1))], 0)
end
% PLOT ZERO-CROSSINGS ON FIGURE TO CHECK: 
hold on
plot(Xzx, repmat(Yd(1,1),1,length(Xzx)), '*r')
hold off

The result:

Since you have the original data and don’t have to get it from the figure, you may want to change the variable designations from my Xd and Yd to yours. If you simply want the X-intercepts (my variable Xzx), then leave it as it is and use (or rename) Xzx as calculated here.

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PhD_77 2014-10-8

Hi Star Strider, Thank you very much indeed. For previous t-y data it was working fine. However I have collected more data and extend t,y plot but it seems the points are not aligned well at y=0.6. Please see the new t-y data (attached). Look forward to hearing from you Many thanks for your help

Star Strider 2014-10-8

My pleasure!

That’s probably as good as it gets. Even when I expand the interpolation to ±50 indices, it doesn’t get more precise. I suspect that with slopes that extreme, you’re also encountering floating-point approximation error. (When I take the mean and standard error of the estimated zero-crossings, I get 0.60014 and 0.000183 respectively.)

My only suggestion is that you increase your sampling frequency by a factor of 10 or more, and perhaps increasing the precision of your ADC as well. That will probably improve the ability of the algorithm to approximate your 0.6 threshold with those data.

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