I recently helped a colleague get the tool tip text from a bar chart so he could display it in another component. I thought I would post what I did in case it could help anyone else in the future. It turned out to be quite simple:
entity = event.source.getEntityForPoint(event.x, event.y)
category = entity.rowKey
series = entity.columnKey
value = entity.dataset.getValue(entity.rowKey, entity.columnKey)
I used this on the mouseClicked and mouseMoved events on the Bar Chart.