怎么定义栅格的行列号吗？例如下面图的最右下边的格子，我想定义为第一行第一列，依次类推。我知道各个网格的中心点的坐标。

这是图的代码
x<-c(0,15)
y<-c(0,15)
par(bg="gray")
plot(x,y,type="n",xlab="",ylab="")
points(c(3,11),c(6,8),pch=21,bg="yellow")
for(x in 0:14){rect(x-.5,.5,x+.5,1.5)
x=x+1}
for(x in 0:14){rect(x-.5,1.5,x+.5,2.5)
x=x+1}
for(x in 0:14){rect(x-.5,2.5,x+.5,3.5)
x=x+1}
for(x in 0:14){rect(x-.5,3.5,x+.5,4.5)
x=x+1}
for(x in 0:14){rect(x-.5,4.5,x+.5,5.5)
x=x+1}
for(x in 0:14){rect(x-.5,5.5,x+.5,6.5)
x=x+1}
for(x in 0:14){rect(x-.5,6.5,x+.5,7.5)
x=x+1}
for(x in 0:14){rect(x-.5,7.5,x+.5,8.5)
x=x+1}
for(x in 0:14){rect(x-.5,8.5,x+.5,9.5)
x=x+1}
for(x in 0:14){rect(x-.5,9.5,x+.5,10.5)
x=x+1}
for(x in 0:14){rect(x-.5,10.5,x+.5,11.5)
x=x+1}
for(x in 0:14){rect(x-.5,11.5,x+.5,12.5)
x=x+1}
for(x in 0:14){rect(x-.5,12.5,x+.5,13.5)
x=x+1}
for(x in 0:14){rect(x-.5,13.5,x+.5,14.5)
x=x+1}
for(x in 0:14){rect(x-.5,14.5,x+.5,15.5)
x=x+1}
x[1]=3
y[1]=6
x2=11
y2=8
m=max(abs(y2-y[1]),abs(x2-x[1]))
n=m+1
addy=(y2-y[1])/m
addx=(x2-x[1])/m
i=2
for(i in 2:n){x=x[1]+(i-1)*addx
y=y[1]+(i-1)*addy
y=round(y)}
data<-data.frame(x=x,y=y)
rect(data$x-.5,data$y-.5,data$x+.5,data$y+.5,col=rev(heat.colors(10)))
lines(c(3,11),c(6,8),type="o",col="green")