Hi Mahmoud,
You can set the value of the F matrix to 0 for the specific region before plotting. You may consider the following code snippet for illustrative purpose:
y = exp(x1);
for i=1:numel(F(1,:))
for j=1:numel(F(:,1))
if i<y(j)
F(i,j)=0;
end
end
end
So, for that specific region the height is zero that should be reflected in plot. If you have follow up questions related to this you may consider contacting MathWorks Tech support of your region and one of the engineer will assist you.