Sine function to calculate y position

In the code below, I have a circle moving up and down, with the function y = sin(x) serving as the origin. However, I need to fix this so that the sine equation is used to only calculate the y-position of the circle at each point in time. I do not want the entire sine wave plotted. What would be the best way to do this? I was thinking a list? Any help appreciated!

x = 0:.01:50; %linspace of x
y = sin(x); %wave equation
px = 10; %initial x plot 
py = 0; %initial y plot
img =imread('AvgBscan.tiff'); %read in image 
for i=1:630 %loop
    imshow(img);
    set(gcf,'DoubleBuffer', 'off');
    h = patch([0 1 0 1], [0 1 1 0], 'r');
    hold on
    figure(100);%so code will replot over the previous figure, and not make a new one.
    hold off
    py = y(i)
    plot(x,y, px, py,'o'); %circle point
    pause(0.05); %speed of moving point
    drawnow       
end

Answer 1

[Rajani Mishra]

Replace line

plot(x,y, px, py,'o');

with

plot(px,py,'o');

this will display just a point rather than the entire sine wave. Also you can refer to below link :

https://www.mathworks.com/matlabcentral/answers/119402-how-to-set-a-marker-at-one-specific-point-on-a-plot-look-at-the-picture