Drawing a line between two dynamic points

2025 3 7
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更新时间：2025 3 8
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Hi
I am making this app, in which i have to draw a line between two points (coordinates). The value of these points keep on changing so with every click a new line needs to be drawn (old one gets removed). You can think of this as a very simple see saw, whose two ends move up and down. I believe that i cannot draw a line on ui.figure, however a line can be drawn on UIAxes, but I am unble to manipulate/translate the two points (coordinates) on UIAxes. Any help in this regard will be greatly appreciated.
Thanks
Rauf

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Jack
Jack 2025-3-7
编辑：Walter Roberson 2025-3-7

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Hey Rauf,
You're on the right track—you can't draw a line directly on a uifigure, but you can draw it on UIAxes. The key here is to update the line every time your points change instead of plotting a new one on top of the old one.
Here’s a simple way to do it:
  1. Use plot to create the initial line in your UIAxes.
  2. Update the XData and YData of the line object instead of re-plotting.
Example Code (App Designer)
function startupFcn(app)
% Initial points
app.P1 = [0, 0];
app.P2 = [1, 1];
% Create the line and store its handle
hold(app.UIAxes, 'on');
app.LineHandle = plot(app.UIAxes, [app.P1(1), app.P2(1)], [app.P1(2), app.P2(2)], 'r-', 'LineWidth', 2);
end
function UpdateLine(app)
% New dynamic points (replace these with your own logic)
app.P1 = app.P1 + rand(1,2) * 0.1; % Example update
app.P2 = app.P2 + rand(1,2) * 0.1;
% Update the line without re-plotting
app.LineHandle.XData = [app.P1(1), app.P2(1)];
app.LineHandle.YData = [app.P1(2), app.P2(2)];
end
% Call UpdateLine() whenever you need to refresh the line
How This Works
  • startupFcn initializes the plot once and stores the line handle.
  • UpdateLine updates the XData and YData properties without redrawing the entire figure.
  • No flickering, no unnecessary new lines stacking up!
If you’re triggering updates on button clicks or real-time changes, just call UpdateLine(app). Should work smoothly! 🚀
If this helps, follow me so you can message me anytime with future MATLAB or Simulink questions! 🚀

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Abdul Rauf Anwar
Abdul Rauf Anwar 2025-3-8
Hi
Thanks for your response. I tried this, but i get this error Unrecognized property 'LineHandle' for class 'app3'. And the properties module of the app looks like this
properties (Access = public)
UIFigure matlab.ui.Figure
UIAxes matlab.ui.control.UIAxes
end
Its all greyed out too, so i am unable to modify/add something to it.
Thanks
Jack
Jack 2025-3-8

The error “Unrecognized property ‘LineHandle’ for class ‘app3’” means you haven’t declared that property in your app. In App Designer you need to add a custom property to store your line handle. You can do this by editing the properties block in your code. For example, add:

properties (Access = public) UIFigure matlab.ui.Figure UIAxes matlab.ui.control.UIAxes LineHandle matlab.graphics.chart.primitive.Line % Add this line end

Now, when you create your line in your startupFcn, assign it to app.LineHandle, and you should be able to update its XData and YData later without error.

Follow me so you can message me anytime with future questions. If this helps, please accept the answer and upvote it as well.

Abdul Rauf Anwar
Abdul Rauf Anwar 2025-3-8
移动：Torsten 2025-3-8
Thanks a lot!
It solved the issue

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