# Drew a circle for a specific point in my plot

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Tiago Dias on 10 Jul 2018
Commented: Tiago Dias on 10 Jul 2018
Hi,
My objective is how to draw a circle for the min(PRESS). I wrote the following code for an ellipse, but I had to adjust the Xcenter and Ycenter handly, but I would rather do this in a automatically form, because if i change the min(PRESS) for example for the first two entries, the values changes and the elipse dont circle the point i want.
[mincumpress,nPC_imp] = min(PRESS(2:end,:))
x_aux = [0:maxpcs]';
figure
plot(x_aux,PRESS,'-b','LineWidth',2);
hold on
plot(x_aux,PRESS,'.r','MarkerSize',15);
yl = ylim; ylim([yl(1) yl(2)]);
xcenter = nPC_imp / xl(2) + 0.035
ycenter = mincumpress * (yl(2) - yl(1)) - 0.08
dim = [xcenter ycenter .05 .05];
annotation('ellipse',dim,'Color','red')
hold on
xlabel('PC Number'); ylabel('PRESS');title('PCA-CV (ckf)');
jonas on 10 Jul 2018
Edited: jonas on 10 Jul 2018
Yea that's correct. Any method where the radius is connected to the axes will give that result, unless the xticks and yticks are equal. Your solution is very practical as the center is connected to the axes while the radius is 'disconnected' from the axes.

KSSV on 10 Jul 2018
Let C be the point, where you want to draw a circle.
N = 10 ;
x = 1:N ;
y = rand(N,1) ;
[val,idx] = max(y) ;
C = [x(idx) y(idx)] ;
plot(x,y) ;
hold on
R = 0.1 ; % Radius of circle
th = linspace(0,2*pi) ;
xc = C(1)+R*cos(th) ;
yc = C(2)+R*sin(th) ;
plot(xc,yc,'r') ;
axis equal
Tiago Dias on 10 Jul 2018
yes, thats what I said to jonas on the previous answer