Hi Jinang,
In your code, the first column of "A" is not a datetime. It is just numeric value calculated by using division operator (/). You should construct two variables: one for datetime, and the other for numeric data. Here is one solution to your problem.
date = [datetime(2019,10,20);datetime(2019,11,12)];
point = [5;6];
table = table(date, point);
plot(table.date,table.point)
