Rotation of a blinking hut about Centre of screen

In this program Hut is rotated about an axis parallel to the axis through one of the points on the hut. See the output to understand more clearly.

#include<graphics.h>
#include<math.h>


float x[5]={100,120,140,140,100};
 float y[5]={100,80,100,140,140};
 int i, j;
 float d2r(float deg){
  float rad = (M_PI*deg)/180.0;
  return rad;
 }
void rotate(float deg){
float rad = d2r(deg);
float x2,y2;
for(i=0;i<=4;i++)
  {
   x2 =(x[i]-320)*cos(rad)-(y[i]-240)*sin(rad) +320;
   y2=  (x[i]-320)*sin(rad)+(y[i]-240)*cos(rad)+240;
   x[i]=x2;
  y[i]=y2;
  }
  clearviewport();
}
void display(int c){
// moveto(x[0],y[0]);
 for(i=0;i<4;i++)
 line(x[i],y[i],x[i+1],y[i+1]);

 line(x[i],y[i],x[0],y[0]);


}
void main()
{
 int gd = DETECT, gm;
 int i;
 initgraph(&gd,&gm,"C:\\TurboC3\\BGI");
 setcolor(3);
 while(!kbhit())
 {
   display(12);
   delay(200);
   rotate(320);
   display(1);

 }
 getch();
 closegraph();


}