If you are refering to the way that a chart will reveal its numeric value while you hover, that is not really a tooltip in the normal sense of layout work.
I don't believe those are controllable. However, I have not worked with 12 charts beyond a few quick experiments so far, but was not able to find any place to control it displaying the actual value on Hover.
Yes, I'm referring to the text that appears next to a data point when you hover the mouse over it. Is that what a layout "tooltip" is?
I have not found a way of changing the text "x value: ..." on the chart.
What I was calling a layout tooltip is a tooltip added via Inspector so while in layout mode (i.e. if a field is empty, telling what type of data it needs). These are programmed as part of the layout design and completely controllable by the developer, though, from what I've seen many developers never use this feature.
I have not found any comparable developer access to or control over what appears when hovering over a chart's data points.