MATLAB: I want to plot a line but got a curve

plot

In my data, t = (0:0.01:0.64)', a is a colunm of constants (-9.81) of length 63. I want to plot a vs. t. It's supposed to be a horizontal line, but I got a graph with fluctuations around -9.81. How can I fix this? I used this code:
plot(t(1:length(a)),a)

Best Answer

t = (0:0.01:0.64)' ; 
a = -9.81*ones(size(t)) ; 
plot(t,a)