make a mandelbrot set

hi, i want to draw the mandelbrot set without the instruction (z=z.^2+c) because i will manipulate the X's and Y's there is my code :

if true
  precision = 0.01;
Nmax = 500;
X=-2.5:precision:1.5;
Y=-1.5:precision:1.5;
[x,y]=meshgrid(X,Y);
xZ=x;
yZ=y;
c=x+i*y;
test=0.;
xC=real(c);
yC=imag(c);
for n=1:Nmax;
xZ=xZ.^2-yZ.^2+x;
yZ=2.*xZ.*yZ+y;
z=xZ+i*yZ;
test=test+(abs(yZ.^2+xZ.^2)<=4);
n;
end;
mesh(X,Y,log(test+10));
colormap(hot.^0.6);
axis([-2.5 1.5 -1.5 1.5]);
axis off;

end

but i have something wrong i don't know what it is ?

please help