I want to plot a line but got a curve

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)

[KSSV]

How did you plot? Show us the code and picture.

[Walter Roberson]

What shows up for unique(diff(a))

[Jiaqi Wang]

Thank you! I think the problem was the calculation kept a long format of the answer of a, which resulted in a minor difference in a.

Answer 1

[KSSV]

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

Answer 2

[Alan Stevens]

Just

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

gives a horizontal line.