Limited number of datacursormode on a scatter plot - Matlab - matlab

Question

I have an ellipse plot (scatter) and I wish to have only two datacursormode and than get their respective X and Y coordinates. The code attached plot the ellipse and mark a point but how can I limit it and get the coordinates?
Code:
clc;
clear;
a=3;
b=7;
x0=0;
y0=0;
t=-pi:0.01:pi;
x=x0+a*cos(t);
y=y0+b*sin(t);
sz = 5;
scatter(x,y,sz)
dcm_obj = datacursormode;
set(dcm_obj,'UpdateFcn',#myupdatefcn)
function txt = myupdatefcn(empt,event_obj)
pos1 = get(event_obj,'Position');
txt = {['X position ',num2str(pos1(1))],...
['Y position: ',num2str(pos1(2))]};
end

Answer 1

You can use a while loop for selecting two points, and use nested functions for sharing variables.
Using nested functions is a possible solution for passing the selected coordinates and a counter out to a parent function.
You can limit the number of selected (marked) points to two, using a while loop.
Placing a pause inside the while is important for responsiveness.
Here is a working code sample:
function ellip()
%Parent function (use a function instead of a script,
%allows sharing variables with internal function).
clc;
%clear;
a=3;
b=7;
x0=0;
y0=0;
t=-pi:0.01:pi;
x=x0+a*cos(t);
y=y0+b*sin(t);
sz = 5;
h = figure; %Open new figure, and keep the handle.
scatter(x,y,sz);
dcm_obj = datacursormode;
dcm_obj.Enable = 'on';
set(dcm_obj,'UpdateFcn',#myupdatefcn)
%pos_x, pos_y and pos_counter are known both in scope of myupdatefcn and in ellip
pos_x = zeros(1,2);
pos_y = zeros(1,2);
pos_counter = 0;
%Loop until two points are selected
while (pos_counter < 2) && isvalid(h)
pause(0.01);
end
if isvalid(h)
close(h); %Close the figure;
%Display position of the two selected coordinates.
fprintf('pos1 = (%f, %f), pos2 = (%f, %f)\n', pos_x(1), pos_y(1), pos_x(2), pos_y(2));
end
dcm_obj.delete() %Delete the object (cleanup)
%myupdatefcn is a nested function, inside ellip function
function txt = myupdatefcn(empt, event_obj)
pos1 = get(event_obj,'Position');
txt = {['X position ',num2str(pos1(1))],...
['Y position: ',num2str(pos1(2))]};
disp(txt);
pos_counter = pos_counter + 1; %Increase counter.
pos_x(pos_counter) = pos1(1); %Store position
pos_y(pos_counter) = pos1(2);
end
end
I hope that I got it right, it is difficult understanding what you are asking...
Update to comment:
Creating the scatter plot outside the function is simple.
Saving the 4 positions is simple - the function can return the points.
I couldn't find an elegant way for leaving the tool-tip information on the plot.
The problem is that calling createDatatip triggers the event and executes myupdatefcn few more times.
You may take a look at Set data tips programmatically?
Updated code:
clc
clear
close all
a=3;
b=7;
x0=0;
y0=0;
t=-pi:0.01:pi;
x=x0+a*cos(t);
y=y0+b*sin(t);
sz = 5;
scatter(x,y,sz);
[pos_x, pos_y] = ellip();
%Display position of the two selected coordinates.
fprintf('pos1 = (%f, %f), pos2 = (%f, %f)\n', pos_x(1), pos_y(1), pos_x(2), pos_y(2));
function [pos_x, pos_y] = ellip()
%Parent function (use a function instead of a script,
%allows sharing variables with internal function).
dcm_obj = datacursormode;
dcm_obj.Enable = 'on';
set(dcm_obj,'UpdateFcn',#myupdatefcn)
h = dcm_obj.Figure; %Get hendle to the figure.
hScatter = findobj(h, 'Type', 'scatter'); %Handle to scatter plot
%pos_x, pos_y and pos_counter are known both in scope of myupdatefcn and in ellip
pos_x = zeros(1,2);
pos_y = zeros(1,2);
pos_counter = 0;
%Loop until two points are selected
while (pos_counter < 2) && isvalid(h)
pause(0.01);
end
% if isvalid(h)
% close(h); %Close the figure;
% end
%dcm_obj.delete() %Delete the object (cleanup)
if isvalid(h)
dcm_obj.Enable = 'off';
set(dcm_obj,'UpdateFcn',[]) %Replace the callback with an empty function instead of deleting the object
end
%myupdatefcn is a nested function, inside ellip function
function txt = myupdatefcn(empt, event_obj)
pos1 = get(event_obj,'Position');
txt = {['X position ',num2str(pos1(1))],...
['Y position: ',num2str(pos1(2))]};
if (pos_counter > 0) && (pos_x(pos_counter) == pos1(1)) && (pos_y(pos_counter) == pos1(2))
return %Return if pod1 is already selected.
end
disp(txt);
pos_counter = pos_counter + 1; %Increase counter.
pos_x(pos_counter) = pos1(1); %Store position
pos_y(pos_counter) = pos1(2);
%Try keeping the tool-tip data
hdtip = dcm_obj.createDatatip(hScatter);
set(hdtip, 'MarkerSize',5, 'MarkerFaceColor','none', 'MarkerEdgeColor','r', 'Marker','o', 'HitTest','off');
end
end