Identifying individual points on scatter plot

Hello,

I have the following problem: I need to crossplot two vectors (x and y) and am using scatter for it. Then I need to be able to identify the index of the point on which the user clicked. So, in short the code looks like this:

---------------------------

h=scatter(x,y,S,C); points=get(h,'Children');
for i=1:numel(children)
set(points(i),'HitTest','on','ButtonDownFcn',{'myFunction',i};
end

---------------------------

I was hoping that the order of children is the same as order of elements in x and y. Looks like it is actually reversed. I could live with this, provided that Matlab guarantees that it will be always this way, i.e. that the relationship between the order of children and the order of x,y elements is fixed now and forever. x and y are not odrered, and searching by their values, which can be obtained from the src argument of the callback, is not very palatable: the vectors may be pretty large.

I'd appreciate any ideas.

Thank you,

Naum Derzhi

[per isakson]

Markup, please!

Answer 1

[per isakson]

I was about to write: See datatip, but found that it is obsolete.

It is replaces by

Data Cursor Text Can Now Be Programmatically Modified

You can now easily customize the text of datatips. The datacursormode function lets you specify the contents and formatting of text displayed by the data cursor tool. When the data cursor tool is active, you can use its context (right-click) menu to edit or specify the text update function that MATLAB executes to display datatips. For more information, see Data Cursor — Displaying Data Values Interactively in the MATLAB Graphics documentation and datacursormode in the MATLAB Function Reference documentation.

Don't know if that is any good in your scenario.

Figure and Axes have the property CurrentPoint

CurrentPoint

two-element vector: [x-coordinate, y-coordinate]

Location of last button click in this figure. MATLAB sets this property to the location of the pointer at the time of the most recent mouse button press. MATLAB updates this property whenever you press the mouse button while the pointer is in the figure window.

All the markers in a scatter-plot is one patch - it seems.

With the axes property CurrentPoint it should be possible. There is no snap AFAIK. There might be problems with points close to each other.

--- @Walter ---

This is my refresh Matlab graphic course :-) I use R2012a and I'm not up to speed with the new stuff that has been added during the last few years. And the old stuff I'm about to forget:-) Thank, you for keeping an eye on my answers.

I was convinced that each point in scatter is a separate object. That was certainly(?) the case once. However, I did the test below, which I think shows that all points are parts of one patch. Scatter did have problems with performance.

    load seamount      % copy&paste from help
    scatter(x,y,5,z)   % copy&paste from help
    h1 = get( gcf, 'Child');
    h2 = get( h1, 'Child' );
    h3 = get( h2, 'Child' );
    disp( [ 'h1 is a ', get( h1, 'Type' ) ] )
    disp( [ 'h2 is a ', get( h2, 'Type' ) ] )
    disp( [ 'h3 is a ', get( h3, 'Type' ) ] )
    whos h* 

    h1 is a axes
    h2 is a hggroup
    h3 is a patch

      Name      Size            Bytes  Class     Attributes
      h1        1x1                 8  double              
      h2        1x1                 8  double              
      h3        1x1                 8  double              

    >> get( h3, {'Xdata','YData'} )
    ans = 
        [294x1 double]    [294x1 double]

    >> whos x y z
      Name        Size            Bytes  Class     Attributes
      x         294x1              2352  double              
      y         294x1              2352  double              
      z         294x1              2352  double

[Naum Derzhi]

I acutally need to do something to other variables in the program depending on the scatter point selected by the user. Which is complicated by the fact that more than one point may have the same x and y, but still differ in other assiciated properties. I hoped to find a way to store a reference to those properties, or their index, with the children of the scattergrpoup, but this does not work.

[Walter Roberson]

Interesting, looks like Mathworks has _already_ redesigned scatter()

Answer 2

[Walter Roberson]

scatter() creates one child per point, and creates them in order of the elements of the inputs. MATLAB always renders children from last to first, so the first in the list is the top-most for renderers that are sensitive to order of operations (OpenGL has its own rules about what will show on top.) You can count on this for all versions from at least 6.5 until R2012a.

However, there is no promise at all that this will not change in future releases. You can be pretty much certain that it will change, eventually, if MATLAB survives as a product long enough.

Mathworks has changed the mechanisms for drawing some objects. For example bar plots used to generate patches as their immediate output, but now bar plots generate barseries objects instead; barseries objects are members of the hggroup class and they have patches as their children (in current versions anyhow.) Patches that are inside a barseries object act differently than direct patches. Anyhow, this is just one of the examples of Mathworks changing how graphics objects are implemented.

If you need guarantees that MATLAB will have some particular behavior "now and forever" then you will probably have to purchase Mathworks (the entire company!) and impose your restriction -- and if you do you should expect that at some point the product line will fail because it has to maintain that behavior when it no longer makes sense to do so in the context of the rest of the product and the rest of the industry. (Mathworks the company is a privately-held company that has never been for sale, but you could probably get an appointment to discuss the matter if you put down a non-refundadable $3.141 billion deposit. )

[Naum Derzhi]

Thank you. What you say is that there is no robust solution, meaning no solution which I can implement and forget. Of course, all development environments keep changing, and no solution survives forever. But if Matlab documented a behaviour, then I could hope that they would also alert the users when the behaviour is changed. Otherwise, they can change it "without notice". Pity.

[per isakson]

One child per point or not? I've added a little test of that to my answer.

[Naum Derzhi]

Yes, it is one child per point. However, you cannot access the children qhile they are being created, only after the whole plot creation. You can access and modify each child after plot creation, but at this moment you do not know which elements of the XData and YData of the whole plot made up this child, especially when you may have several points with the same X-Y coordinates, but different in a number of other aspects. You can only hope that the "rule" that the children are created in the order of the elements in X, Y (a very reasonable assumption, which I believe will hold true forever), and that they are returned by get('Chidren') in reverese order will hold. The latter assumption will probably fail in some future relase, as Waleter Roberson pointed out.

[per isakson]

I'm confused regarding "one child per point". I use R2012a. Have you executed the test with the seamount data, that I added to my answer? If so, do you get the same result? Do I draw the wrong conclusion from the results of the test? Or does the test miss the point?