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How do I find the index of a datapointon a MATLAB plot?

Asked by S. Moore on 24 Aug 2016
Latest activity Commented on by Hassan Nikoo on 20 Aug 2018
With a very large dataset - assume 1 million [x,y] pairs or more - how do I find the index of an individual datapoint on a MATLAB plot?
Example: I use plot(x,y) and the plot appears. The plot is complicated but there are artifacts I can locate by eye much easier than writing a detection script. I zoom into the plot and locate the desired data artifact. I need to know the exact location (index) of the artifact. I zoom that individual datapoints are apparent, place a cursor on the datapoint of interest, but the cursor does not report the exact x(index) of the datapoint. Other than manually scrubbing through the data to find the index, how do I find the index by using the plot?

  1 Comment

Why not use find() once you have the xy values?

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2 Answers

Answer by D. Plotnick on 12 Jul 2018

This is my kludgy workaround: instead of
plot(x,y)
do
plot3(x,y,1:length(x));
view(0,90)
NOW use your data cursor, the z-value is the index of the point you are currently attached to. Voila!
I do wish this was an easier/more elegant change to the DataTip tool, since it would also be useful to use a standard plot command but with a third parameter variable that does not get plotted but does show up in the tooltip; useful for parametric equations. Something like my made-up 'plotParametric':
theta = linspace(0,2*pi,101); R = 1;
[x,y] = pol2cart(theta,R);
plotParametric(x,y,theta);
where now the tooltip will also display the parameter value theta at a given x-y point. Replace this with index, and it is your above problem.

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This is GREAT. Great idea thanks.

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Answer by Julian
on 25 Aug 2016

Surely the approved method for this is to use a data cursor? I fear that it may respond slowly to navigation if you have a million points in the plot though.
You can try it with an elementary plot first to get the hang of it
plot(randn(100,1))
datacursormode % or you can use the icon in the figure window
shg

  3 Comments

No. When I click on the artifact, the data cursor will display the [x,y] vaules. The X value is displayed 8.946E+05 instead of 895,631.
Furthermore, adjusting the data type in the console (i.e. "long") does not change the data display on the cursor.
I find that a problem too sometimes!
You could write your own DataTip callback functions to customize the display - but that's somewhat more work.
You can export the cursor to the workspace to discover its precise value - that is what I often do, although I think it's more of a faff than it needs to be.

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