There might be a situation, when you need to show balloon over some data point without a mouse, but based on user interaction outside the chart.
For pie chart, this is quite straight forward, it has rollOverSlice(index) method. So if you know the number of a slice, you simply call this method. In case you have the title name of a slice, you should loop through your data, find matching data item and then call rollOverSlice method. Here is jsFiddle illustrating this.
For Serial chart is not as straight forward, but also easily doable. First, you have to add ChartCursor to the chart. And then you simply call chartCursor.showCursorAt(category) method. Category here should be a string, matching on of your category or a date object, if your chart parse dates. And here is a sample.
In case you need this text annotation to be always visible, you can use Stock charts events for this purpose.
You can do this also with our maps. AmMap has a method, map.rollOverMapObject(object). Here is a working sample where roll-over on the map is triggered when user rolls-over the object on line or bar chart.