Hey,
Notice that you are always updating a new value of x and y at each iteration, so in the end you have not a pair of vectors with values but just one pair of values.
Maybe something more like this?
y(1) = 0;
x(1) = 0;
i = 1;
e=2.718;
while y <= 9.8
y(i+1) = y(i) + (10*(1-e^(-x(i)/4)));
x(i+1) = x(i) + 1;
i = i + 1;
end
plot(x,y)
Greets
