How can I infer from the data cursor position, the index of the closest datapoint

Hi everyone,

I am working in the App Designer and I want to have a tool which lets the user select a bunch of datapoints. I have a UI Axes and defined a ButtonDownFcn.

Once the user clicks in the axis I basically want to trigger some sort of selection process. The goal is to select the datapoints from the first click up until the mouse is clicked a second time. It is not obvious to me how I can infer from the cursor position (which i can extract) which of my datapoints is closest. Lets say I have plotted 1000 random datapoints, I dont know how a cursor position of say 200 could give me any idea about my real values.

Maybe some of you have encountered a similar problem and can help, that would be really great. Also if there may be a better solution, I am happy to hear it.

Thank you very much!!

For the button down function i was thinking about something along the line of:

selectedRegions = [];

% Button down function: RawEEG_ax

app.ax.ButtonDownFcn = @(hObject, evt) axclick(hObject,evt);

function axclick(hObject,evt)

x = dataIdx1;

cp = hObject.CurrentPoint;

cx = cp(1,1);

if isempty(selectedRegions)

selectedRegions(1) = round(interp1(x, 1:numDataPoints, cx, 'nearest', 'extrap'));

else

selectedRegions(2) = round(interp1(x, 1:numDataPoints, cx, 'nearest', 'extrap'));

end

end

I know there is also this datacursor mode, but I was hoping to find a better solution than using this, since I actually do not just want the indexes of te datapoints, but was hoping to have some more functionality once I have solved this first problem, e,g, live tracking of the cursor to visualize the selected region and I think for that purpose the datacursor mode is not optimal, so I wanted to try something else.

Answer 1

[Allen]

Julian,

What you are asking for sounds very much like what the brush data tool does. However, there are a few extra steps required to extract the data from the brushed data points. The following links provide guidance on this tool and how to extract data.

Brush Data Tool

https://www.mathworks.com/help/matlab/ref/matlab.graphics.interaction.internal.brush.html

Extracting data values from brushed points

https://www.mathworks.com/matlabcentral/answers/1966569-extract-brushed-data-programmatically

https://www.mathworks.com/matlabcentral/answers/505340-how-to-get-brushed-data-point-index-or-coordinate-from-a-3d-plot#answer_418629

[Julian Ostertag]

Hi Allen,

thank you very much. Indeed it sounds like it would fit. However it did not work for me (but probably because my skills were not good enough). However I found when the xaxis is simply a vector from 1 to N, with N being the number of elements, then one can easily infer from the cursor position the x value. I had a more complicated xaxis before and therfore missed this. Thank you anyway :)