[charts] Balance charts should only have a datapoint when a transaction occurred

Currently the chart displays the balance amount with a point for every single day. This gives the impression that each of these days a transaction occurred, which is false.

We should set a point only if the amount differs from the previous day, or better: if there was a transaction that day (which is different if during a day there are both 1 positive and 1 negative transaction from the same amount).

Bonus: it would make the chart's drawing quicker.