Highlight "Billable" attribute on timeline
There is no way to quickly recognize billable time on Tags timeline. Auto-tags can display a red streak for auto-closed tags. Use the same visual mechanism on tags timeline denoting billable activities. (Billable? Tag has a green streak.)
(The visualization may be optional as someone may not like it.)
I agree with both the requirement and the proposed solution. Seems important and probably relatively easy to implement.