请高手帮忙，如何使我的图能画出半圆环的效果
我之前的程序如下：
dX = (pi/2-(-pi/2))/10
dY = (Ro-Ri)/10
X(1) = - pi/2;
X(2)=dX/2+X(1);
for i = 3:ni-1
X(i)=X(i-1)+dX;
end
Y(1) = Ri;
Y(2)=dY/2+Y(1);
for j = 3:nj-1
Y(j)=Y(j-1)+dY;
end;
Y(nj)=Ro;
for i = ni-1:-1:2
T(i,j) = P(i)*T(i+1,j)+Q(i);