Updating Discountinuous Plot Lines Using Handlers

Hey people!

Recently I needed to plot a bunch of discountinuous lines in a plot using a single call to the function. In this post ( https://la.mathworks.com/matlabcentral/answers/425844-draw-a-discontinuous-line ), its said that if you have two lines:

1. Line 1: Starts in (x1, y1). Ends in (x2, y2).
2. Line 2: Starts in (x3, y3). Ends in (x4, y4).

Then you can put create two arrays with the following structure

XCoords = [x1 x3; x2 x4];
YCoords = [y1 y3; y2 y4];

And then simply type

Lines = plot(XCoords, YCoords) 

This solution worked great when I tried it on a static plot with 10 different lines, however, I need to dynamically update the XData and YData properties for an animation. The thing is, when I try to assign new values to the X and YData

Lines = plot(XCoords, YCoords)          % This Works. XCoords / YCoords dims: 2 X 10 Matrix

Lines.XData = XCoords;                  % Error. PlotDataX dims: 2 X 10 Matrix

I get the following error: "Expected one output from a curly brace or dot indexing expression, but there were 10 results". I thought this came from a dimension mismatch so I printed the values from the plot XData and got the following output (.mlx file output).

ans = 1x2
    x1  x2
ans = 1x2
    x3  x4
...
ans = 1x2
    x19 x20

It seemed like a dimension mismatch, so I tried the following fix

NewPlotDataX = XCoords(:)';         % NewPlotDataX dims: 1 X 20 Matrix
[Lines.XData] = NewPlotDataX;

Both vectors have exactly the same dimensions (1x20), but even then I get a new error: "Insufficient number of outputs from right hand side of equal sign to satisfy assignment". I don't know what to do anymore. Any fix for this? Maybe an alternate way to animate a bunch of discontinuous lines using a single plot command and handlers?

Answer 1

[Eddy Sanoli]

Thank you Walter! At the end I decided to use quiver() to draw the discontinous lines. When updating the handlers you just update the "Plot.UData" with the first point of all lines and the "Plot.VData" with the second point of all lines. The only extra step is to set the 'ShowArrowHead' property to 'off' to get a line instead of an arrow. This was the most efficient, simple and clean method I found.

[Walter Roberson]

quiver() sounds useful for this situation; good idea.

[Eddy Sanoli]

Other possible solution that I just discovered is separating the "connected" line sections with NaNs. In this way the plot function only connects points not separated by NaNs

Answer 2

[Walter Roberson]

Lines = plot(XCoords, YCoords) 

When either XCoords or YCoords (or both) are 2D (not vector), then some size matching will be done to make sure you have the best orientation for X relative to Y. If X is then 2D but Y is a vector, Y will be replicated to the same number of columns as X has. After that Y will be a 2D array, and the output, Lines, will contain one line() object for each column of Y.

Lines.XData = XCoords;

That is an error because Lines is more than one line() object. You cannot use dot notation to update more than one object.

In order to update more than one object in a single call, you need to use set(), and you have to be quite careful with the syntax of the set() call; I am having a difficult time getting it to work at the moment.

I advise you to loop setting corresponding coordinates, or use a hidden loop such as cellfun(). You will need to break it up per object (even if that just involves using num2cell()): you will definitely not be able to set the coordinates of all of the objects from a single numeric matrix.