To set this up, you'll need the following strategies:
Creating a Smart List
The first step is to create a smart list that will be used to select items that have tracked time. To do that, we'll use isTracked advanced filter. To create a smart list, click on the ☰ icon in the top-right, and go to Smart Lists. There create a new smart list using isTracked advanced filter that you can find at the bottom, under all other filters:
Keep in mind that this filter is still experimental so some items might not end up on the list.
Creating a Line Marker
Now that you've created the smart list, by going to the ☰ 🡒 Strategies 🡒 Line Marker you can add a new marker and connect it to this smart list. By clicking on the dot next to the marker's title, you can change its color.