Map the Cursor.Position property to your chart. An example with a panel:
private void panel1_MouseHover(object sender, EventArgs e) {
var pos = panel1.PointToClient(Cursor.Position);
toolTip1.Show("Hello", panel1, pos);
}
Note that this is not otherwise different from using toolTip1.Show("Hello", panel1); but you are likely to want to tweak the tooltip position so it isn't overlapped by the cursor.