How can I display Index of a data point on the Data Cursor?

When I use the Data Cursor option on a plot, I am able to view the X and Y-coordinates on the tool tip. However I wish to see the index of that point. I have tried editing the textUpdate function as follows:

function output_txt = myfunction(obj,event_obj)

% Display the position of the data cursor

% obj          Currently not used (empty)

% event_obj    Handle to event object

% output_txt   Data cursor text string (string or cell array of strings).

pos = get(event_obj,'Position');

x = pos(1); 

y = pos(2);

idx = find(xydata == x,y);

[row,~] = ind2sub(size(xydata),idx);

output_txt{end+1} = cell2mat(labels(row));

However this gives an error. Does anyone have any suggestions?

Thanks in advance

Deepa

[Adam]

You haven't shown what you are doing with that function (how you are calling it) or what the error is so you can't really expect to get too much help if you don't give all relevant information.

dcm = datacursormode( hFig );
dcm.UpdateFcn = @updateDataCursor;

is the kind of thing to use to create a custom data cursor if you put the relevant code in updateDataCursor which may be what your 'myFunction' is, but that would just be a guess since you haven't shown that part of the code.

[Deepa T]

You are correct. The updateDataCursor is myFunction. I simply edited the default MATLAB function for data cursor.

By default, the data cursor displays x coordinate and y coordinate on the tooltip when a data point is clicked. I wish to display the index of the data point that is being plotted instead.

I have since made some modifications to the code. Code in main pgm:

dcm_obj = datacursormode(fig);

set(dcm_obj,'UpdateFcn',@myupdatefcn);

The updatefunction code is:

function txt = myupdatefcn(obj,event_obj)

pos = get(event_obj,'Position');

x = pos(1); y = pos(2); I = get(event_obj, 'DataIndex');

txt = {['X: ',num2str(x)],...
       ['Y: ',num2str(y)],...
       ['I: ',num2str(I)]};

Currently the DataIndex displays the same value as X.

X:8 Y:0.6 .DataIndex:8

I need it to display the index of the Yth variable. For example, if the point I have selected (8,0.6) is at index [8,59], I need it to display:

X:8 Y:0.6 DataIndex:59

Hope this clarifies the confusion.

Deepa

Answer 1

[Deepa T]

I've received the desired output with the following code:

function txt = myupdatefcn(~,event_obj,myarray)
pos = get(event_obj,'Position');
ind = find(myarray(:,pos(1))== pos(2));

txt = {['X: ',num2str(pos(1))],...
       ['Y: ',num2str(pos(2))],...
       ['Index: ',num2str(ind')]};
end

as well as the following code in the main function.

dcm_obj = datacursormode(fig);
set(dcm_obj,'UpdateFcn',{@myupdatefcn,myarray})

Many thanks to everyone who helped.

[Samuli Karvinen]

Hello,

altought this is an old post I still need to ask. What is this "myarray" function you have added as an input? I'm also trying to get index for my data points without any result..

[Deepa T]

myarray is the name of the dataset I'm using. It's not a function.

[Samuli Karvinen]

Hey,

Oh I see, what data have put in there? The data tip objects?

Thanks in advance!

Samuli

Answer 2

[Les Beckham]

Try this (modify the num2str calls as you wish to get the number of digits you desire):

function output_txt = datatip_cb15digits(obj,event_obj)
% datatip_cb15digits.m - datatip callback function - modified to display 15 digits
% Display the position of the data cursor
% obj          Currently not used (empty)
% event_obj    Handle to event object
% output_txt   Data cursor text string (string or cell array of strings).

pos = get(event_obj,'Position');
handles = get(gca, 'Children');
h=handles((handles == event_obj.Target));
dim = 2;    % initially assume this is a 2d plot

output_txt = {['X: ',num2str(pos(1),15)],...
    ['Y: ',num2str(pos(2),15)]};

% If there is a Z-coordinate in the position, display it as well
if length(pos) > 2
    output_txt{end+1} = ['Z: ',num2str(pos(3),15)];
    dim = 3;
end

% display the index of the currently selected datapoint
if (dim == 2)
    i = find(get(h, 'XData') == pos(1) & get(h, 'YData') == pos(2));
else
    i = find(get(h, 'XData') == pos(1) & get(h, 'YData') == pos(2) ...
        & get(h, 'ZData') == pos(3));
end
output_txt{end+1} = ['i: ', num2str(i)];

[Les Beckham]

Some more research reveals that you can get the index as follows:

i = get(event_obj, 'DataIndex')

Maybe that property was added since I created this function in 2008.

I learned something new today!

[Deepa T]

I tried this function too. My issue is that it displays the Index value along the X-axis. I need to display the index of the Y-axis.

I'm attaching an image of how it's currently being displayed:

But I wish to see the index corresponding to the Y-axis.

I'm wondering if that's possible. Am attaching my current code also.

dcm_obj = datacursormode(fig);
set(dcm_obj,'UpdateFcn',@myupdatefcn);

 function txt = myupdatefcn(obj,event_obj)
  pos = get(event_obj,'Position');
  x = pos(1); 
  y = pos(2); 
  I = get(event_obj, 'DataIndex');
  txt = {['X: ',num2str(x)],...
         ['Y: ',num2str(y)],...
         ['I: ',num2str(I)]};

[Les Beckham]

I don't understand what you mean by "the index corresponding to the Y-axis". In an x-y plot each specific point on particular line has only one index and corresponding x and y values. Perhaps you are trying to determine which 'line' a particular point corresponds to?

It would help if you show a simplified example of how you are plotting your data and what you mean by the "index of the Y-axis".

[Walter Roberson]

Correct, the data index is specific to the line that was clicked on, and indicates the relative offset into the line, as in XData(index) YData(index) . This need not be sorted by x or y, and is completely independent of every other line or graphics object.

If you have access to all of the data to be plotted, you could always unique() the complete list of y values concatenated together, and request the third output of unique() so you know the index of each of the values relative the list of unique values, and then you could break that list up using mat2cell() according to the length of each original line, and then you could set() the UserData for each line to include the y indices for each point. Then your update handler can use the data index to index the UserData to get the information to display.

[Deepa T]

Thank you, Walter. That sounds like what I want to do. I'm trying to implement the code for this. Afraid I'm a newbie to MATLAB :(

Thank you

Deepa

Answer 3

[Walter Roberson]

Suppose you have a cell, Xs, of all of the X values, and a cell Ys, of the corresponding Y values, such as might be plotted with

XYs = [Xs(:).'; Ys(:).'];
plot(XYs{:})

Then,

%create a giant column vector of all of the Y values
%and find the unique Y values and the indices of each
%original value into the unique list.
%this code does not assume anything about the orientation
%of the vectors
[uY, ~, uYidx] = unique( cell2mat( cellfun(@(V) V(:), Ys(:), 'Uniform', 0) ) );

Ylens = cellfun(@length, Ys);
Yidxcell = mat2cell(uYidx, Ylens, 1);

XYs = [Xs(:).'; Ys(:).'];
linehandles = plot(XYs{:});
for K = 1 : length(linehandles)
  set(linehandles(K), 'UserData', Yidxcell{K});
end

Together with

function txt = myupdatefcn(obj, event_obj)

pos = get(event_obj,'Position');

x = pos(1); y = pos(2);
I = get(event_obj, 'DataIndex');
Yidxs = get(obj, 'UserData');
if isempty(Yidxs) || length(Yidxs) < I;
  Yidx = '?';
else
  Yidx = Yidxs(I);
end

txt = {['X: ',num2str(x)],...
       ['Y: ',num2str(y)],...
       ['I: ', Yidx]};

[Les Beckham]

There are definitely some very clever Matlab tricks and manipulations going on here, but in what legitimate use case would anyone ever create a plot this way? Since the OP never gave us an example of how the data is being plotted or what they are really trying to achieve, I'm skeptical that your very creative answer will really solve the poorly defined problem. OP did comment that "That sounds like what I want to do," so, hopefully I'm wrong.

I would definitely be interested in some sort of real world example where this approach would apply.

[Walter Roberson]

Les, see https://www.mathworks.com/matlabcentral/answers/347916-i-have-an-array-which-is-increasing-at-one-point-it-starts-to-decrease-this-trend-starts-repeatin#comment_467698 where I implemented a plot using exactly this scheme of cells of coordinates.

Answer 4

[Seyhan Özen]

another working solution:

https://www.mathworks.com/matlabcentral/fileexchange/29163-index-or-subscript-in-datatip