ginputplot

 GINPUTPLOT finds points on the plot nearest to the mouse click
 
   [XI,YI] = GINPUTPLOT(X,Y,N) gets N points from the current axes and 
   returns the nearest points on the plot of Y versus X, in length N 
   vectors XI and YI. For further details see ginput.
   [XI,YI] = GINPUTPLOT(X,Y) gathers an unlimited number of points until
   the return key is pressed.

 Example:
 [t,Y] = ode23(@lotka,[0 5],[20 20]');
 x = Y(:,1);  y = Y(:,2);
 plot(x,y)
 [xi,yi] = ginputplot(x,y)