problem with matrix rotation

hi to all, i have a problem when i try to rotate a line or an object the problem is centre of rotation that is not what i want. with teta=0 i have a line that starts at (15,15) and go to right. instead if i use teta=20 i want i line that start always in (15,15) with an angle of 20 degree but the start point change.WHY?????? i use tha rotation matrix. can somebody help me please?????? here is my code

      x0_center=0;
      y0_center=0;

     w=30;
     ha=30;
     teta=0;
     %r=rectangle('Position',[x0_center,y0_center,w,ha],       'LineWidth',3,'LineStyle','--');
figure;
DrawRectangle([x0_center,y0_center,w,ha,teta]);
axis([0 40 0 40])
axis equal
%set(r,'edgecolor','r')
%xlim([0,25])
%ylim([0,15])
hold on

 x0=15;
 y0=15;
 x=20;
 y=15;
 n=10;

 k=1;
            teta=20;
            xvet=linspace(x0,x,n);
            yvet=linspace(y0,y,n);
        P = [xvet;yvet];
         theta=(teta*pi)/180;

        ct = cos(theta);
        st = sin(theta);
        R = [ct -st;st ct];
        P_t = R * P;
        xvet_t=P_t(1,:);
        yvet_t=P_t(2,:);

            for index=1:10
            xvet1(index)=xvet_t(index);
            yvet1(index)=yvet_t(index);

            h1(k) = line(xvet1, yvet1,'EraseMode','none');
            set(h1, 'XData', xvet1, 'YData', yvet1);
            pause(0.05);

        end
    x0=x;
    y0=y;
    xvet = NaN(1, n);
    yvet = NaN(1, n);
    xvet1 = NaN(1, n);
    yvet1 = NaN(1, n);

Answer 1

[Matt J]

Your current code performs a rotation about the origin, wherever that happens to be. To specify [15,15] as the center of rotation, you can use FEX:AxelRot as follows

 P(3,:)=0; %embed in 3D

 P_t=AxelRot(P,20,[0 0 1],[15,15,0]);  %perform the rotation about [15,15,0]

and then the rest of your code.