I'm not quite clear on what you are trying to accomplish but it looks like some how you want to see the plot continue to evolve as time goes along.
You could modify your code as follows and see the points continuously updated, the key is to use the hold on command:
numIter = 1000;
tOld = datetime('now');
yOld = rand(1);
for k = 1:numIter
y = rand(1);
tOld = t;
yOld = y;
Note, I made the code loop for a fixed number of iterations rather than have to ctrl-c to end the infinite while loop that you had.
Also a few other cleanup items, the clock command returns a six element date vector. If you call datetime(clock) it must first convert the clock value to a datetime. Instead just use datetime('now'). There is no need for formatting the value returned by calling datetime('now'), that is handled by the xtickformat
In the above code I connected the data points with lines. If you don't need the dots connected by lines then you don't need to save the previous values, you can just plot t and y with a symbol