How to interactively select different plots in a figure and merge them together?

I have several little lines of plot that represent portions of particle trajectories: each of such "pieces" are described by x- and y-coordinates. I am looking for a way to produce a figure of all these pieces, click on each of the pieces that visually belong to the same trajectory, and merge their x- and y- coordinates into a single array.

Here is a very simple example:

x = linspace(0,pi,100);
y = sin(x); z = cos(x/2);

x1 = x(1:12); y1 = y(1:12);
x2 = x(19:25); y2 = y(19:25);
x3 = x(45:67); y3 = y(45:67);
x4 = x(90:end); y4 = y(90:end);
x5 = x(4:19); y5 = z(4:19);
x6 = x(40:88); y6 = z(40:88);

plot(x1,y1,'b'); grid on; hold on
plot(x2,y2,'b')
plot(x3,y3,'b')
plot(x4,y4,'b')
plot(x5,y5,'r')
plot(x6,y6,'r')

From here, I need to click on all the blue plots (which belong to the "sine" trajectory) or red plots (which belong to the "cosine" trajectory), collect their coordinates and save them into a single array (for a future reconstruction of the original trajectory or whatever).

Searching a bit through MATLAB Answers, I came across this question that explains how to extract data from MATLAB figures. This is quite similar to what I need; but instead of by inspecting properties, I need to do it interactively by clicking on each plot, one after the other, and saving in the workspace the XData and YData of all plots I have clicked. In this sense, the concept should be similar to what ginput does with plot coordinates.

Answer 1

[Sachin Lodhi]

Hi Lorenzo, from what I understand, you want to join together line segments on a figure in a way that you can control as you go. Below, I've put together a piece of code to help you start with this. Remember, this is just a basic setup for merging lines, and you might need to tweak it a bit to make it work just right for you.

function merge_plot_pieces
    % Sample plot pieces
    x = linspace(0,pi,100);
    y = sin(x); z = cos(x/2);

    % Define segments
    segments = { x(1:12), y(1:12); x(19:25), y(19:25); x(45:67), y(45:67); x(90:end), y(90:end); x(4:19), z(4:19); x(40:88), z(40:88)};
    colors = ['b', 'b', 'b', 'b', 'r', 'r'];

    % Initialize global variables to store merged coordinates and last color
    global mergedX;
    global mergedY;
    global lastColor;
    mergedX = [];
    mergedY = [];
    lastColor = ''; % Empty indicates no color selected yet

    % Plot segments
    fig = figure; hold on; grid on;
    for i = 1:size(segments, 1)
        h = plot(segments{i,1}, segments{i,2}, colors(i));
        set(h, 'ButtonDownFcn', @plotClicked, 'UserData', colors(i)); % Store color in UserData
    end
    
    % Set up a key press function to reset the merging process
    set(fig, 'KeyPressFcn', @resetMerging);
end

function plotClicked(src, ~)
    % This function is called when a plot segment is clicked
    global mergedX;
    global mergedY;
    global lastColor;

    currentColor = src.UserData; % Get the color of the clicked plot from UserData

    % Check if the color matches the last clicked color or if it's the first click
    if isempty(lastColor) || strcmp(lastColor, currentColor)
        % Append clicked segment data to global variables
        mergedX = [mergedX, src.XData];
        mergedY = [mergedY, src.YData];
        lastColor = currentColor; % Update lastColor

        % Plot the merged trajectory
        plot(mergedX, mergedY, currentColor, 'LineWidth', 2, 'Tag', 'mergedTrajectory'); 
        legend off 
    else
        % If the colors don't match, do not merge and optionally alert the user
        disp('Cannot merge: Selected segment is of a different color.');
    end
end

function resetMerging(~, ~)
    % This function resets the merging process to allow starting with a new color without clearing the last merged trajectory
    global mergedX;
    global mergedY;
    global lastColor;

    % Reset the global variables without clearing the last merged trajectory
    mergedX = [];
    mergedY = [];
    lastColor = '';
    
    disp('Merging process reset. You can now start merging a new color.');
end

I hope this helps.

[Lor M]

Hi @Sachin Lodhi, thanks a lot for your help! You understood correctly: your code does exactly what I wanted. Also thanks for directing me to the use of the ButtonDownFcn property: I was aware of its existence and suspected that I could use it to fulfill my task, but I always find hard to understand how to use it.

However, I have experimented a bit with your code and cannot get to save the merged trajectories (mergedX and mergedY) in the workspace. In my understanding of how your code works, it seems that mergedX and mergedY "live" within the merge_plot_pieces space and are not loaded into the main workspace when, e.g., a button is pressed to trigger the resetMerging. Since merge_plot_pieces is a function, I thought I could manage with changing the function definition like

[mergedX,mergedY] = merge_plot_pieces

but it is not working. Any idea on how to do that?

[Sachin Lodhi]

To save the merged trajectories (mergedX and mergedY) in the MATLAB workspace from within the function, you need to use the 'assignin' function. The 'assignin' function allows you to assign a value to a variable in a specified workspace, which in your case would be the 'base' workspace (MATLAB's main workspace).

You can modify the plotClicked and resetMergign function in the following way -

function plotClicked(src, ~)
    global mergedX;
    global mergedY;
    global lastColor;

    currentColor = src.UserData; % Get the color of the clicked plot from UserData

    if isempty(lastColor) || strcmp(lastColor, currentColor)
        mergedX = [mergedX, src.XData];
        mergedY = [mergedY, src.YData];
        lastColor = currentColor;

        plot(mergedX, mergedY, currentColor, 'LineWidth', 2, 'Tag', 'mergedTrajectory');
        legend off 

        % Save to base workspace
        assignin('base', 'mergedX', mergedX);
        assignin('base', 'mergedY', mergedY);
        assignin('base', 'lastColor', lastColor);
    else
        disp('Cannot merge: Selected segment is of a different color.');
    end
end

function resetMerging(~, ~)
    global mergedX;
    global mergedY;
    global lastColor;

    mergedX = [];
    mergedY = [];
    lastColor = '';

    % Save the reset state to base workspace
    assignin('base', 'mergedX', mergedX);
    assignin('base', 'mergedY', mergedY);
    assignin('base', 'lastColor', lastColor);
    
    disp('Merging process reset. You can now start merging a new color.');
end

[Lor M]

@Sachin Lodhi, again thank you very much for your kind help! assignin was just the missing piece I needed.