You can add annotations to your Charts and Graph drawing in Visio.
On the Shapes pane, from Charting Shapes, drag any of these shapes on to the drawing page — 1-D word balloon, 2-D word balloon, Horizontal callout, or Annotation.
With the shape selected, type the text you want.
Drag the control handle or endpoint at the end of the pointer to change the pointer’s length or direction.
Tip: You can glue the endpoint of the annotation to a connection point on the chart or graph so that the annotation stays connected if you move the shape. To do this, select the entire shape — not a subshape — and then add a connection point. Drag the endpoint of your annotation shape to the connection point. If you resize the chart or graph, you may need to move the connection point.