Chart annotations allow users to add notes to charts as well as add them to global groups to share across multiple charts.
First, annotate the Revenue Per Day chart to show when the android and iOS releases were.
Click a data point to add a note at that point:
Click Add to Group to open up the group options menu. Select New Group to create a new group for Launch Dates:
After adding the android launch date, the chart looks like this:
Next, add the Launch Dates group to the Daily Returning Users chart. Head over to the Chart Format Tab within the Chart Editor:
On the lefthand side under Annotation Groups, select Launch Dates:
Now the annotations are shared across the Revenue Per Day and Daily Returning Users charts.
Finally, delete a misplaced annotation. Start by hovering anywhere over the annotation line.
Hover over the annotation text until the 'x' appears. Click the 'x' and the annotation is crossed out.
Click the 'x' again to undo the deletion or click anywhere off the chart and the annotation will be deleted.