Task Burndownchart

There's nothing in Scrum which says that tasks must be estimated, or indeed that tasks be used at all when forming a Sprint Backlog. They are simply a convention and estimating them is also a convention.

What is important is that transparency is provided over the work remaining in a Sprint. As the Guide says, "At any point in time in a Sprint, the total work remaining in the Sprint Backlog can be summed". Estimation of tasks is one way to facilitate this, but there are other means. One of the simplest is task counting, where the burndown merely registers each task as it is completed. In effect, each task is estimated as being 1 (unity). This can also have the advantage of encouraging a finer granularity of task planning.

I agree with Ian.

In my team we divide the story points based on the overall weight of the (sub)tasks. So if I have a story of say 8 points and we have discovered 5 tasks, we try to see if there are tasks that are bigger/smaller than others and divide the points say like [2 points, 3 points, 1 point, 2 points]. This allows us to see how the story is progressing batter than purely task counting.

But see what works for your team and stick with that.