dpb is close, but not quite there.
The problem is when x == 0. Look at your code, line by line, thinking about what happens. (This is how you should debug these issues, and it took me far less time to resolve the issue than it took me to write this response.)
When x == 0, we see that a == 0.
Likewise, when x == 0, b == 0, and therefore c == 0.
And if you look at d, clearly when both c and x are zero, it too will be zero.
So now look at e. What is 0/0? NaN. And NaNs are like rabbits - they propagate.
Therefore, when x == 0, you get z == 0 along that line.
The trivial fix is to avoid having x == 0. So instead of having a spacing of 0.01, which gives you an odd (201) number of points over the interval [-1,1], use exactly 200 points.
[x,y]=meshgrid(linspace(-1,1,200));
The disfiguring line of NaNs will now be gone.
