怎么用C语言作抛物线，急！！！！！！！

/*这是一个画抛物线的程序*/

#include<graphics.h>

#define BKC 9
#define COLOR 8 /* 抛物线颜色*/
#define G 9.856

main()
{int gd=DETECT,gm,p=1,i;
double Vx,Vy,x,y,x1,y1,t;
char ch;
initgraph(&gd,&gm,"");

while(p==1) /*主循环体*/
{
setbkcolor(BKC);
setcolor(14);
setlinestyle(0,0,1);
rectangle(0,0,639,479); /*画边框*/
line(20,460,639,460); /*画横坐标*/
line(20,0,20,460); /*画纵坐标*/
outtextxy(8,467,"0");
setlinestyle(1,0,1);
setcolor(15);
for(i=20;i<640;i+=20)line(i,460,i,0);
for(i=460;i>0;i-=20)line(20,i,640,i);
gotoxy(1,1);
printf("Please Enter Vx:" );
scanf("%lf",&Vx);
printf("Please Enter Vy:");
scanf("%lf",&Vy);

for(t=0.0;;t+=0.00002)
{
x=Vx*t;
y=Vy*t-0.5*G*t*t;
x1=2*x+20; /*画笔定位在用户坐标原点 (20,460