SYMPTOM:
When executing a Report Services Document in Interactive mode in Strategy Web 9.x the Tooltips are not displayed when the pointer is hovering over a series in a line graph. In this example below, the mouse is hovering over the Cost metric series in blue.

CAUSE:
This is working as expected. For the tooltip to show the value of a specific point in a series, the Data Labels or the Data markers should be enabled for the series.
ACTION:
In Strategy Desktop, edit the Graph Report, go to Graph view, select the series, give a right click and select Series Options...
Either check the box for 'Show Data Marker' under the General tab:


Or check the box for 'Show the Data Labels' under the Data Labels tab:

