Hi Laia,
I see you use reactive.
there is a default tooltip created by the chart widget, but you can use the chart widgets tooltip property to make one according to your own liking, for example including a currency symbol.

See attached oml for an example
I'm sure there is also a way to do this with JSON, but this seems like the way to go for me.
Dorine
Edit : I just saw that you are talking about a donut chart, my example is a column chart, but same thing applies.