Adding dependencies between task lists on the diagram view is straightforward. Just use your finger to draw a line from an arbitrary task to another task in a different list, indicating that work on the second task cannot start until the first one has been completed.
The line will change its color to indicate the dependency you would like to add is valid (green) and can be added, an identical dependency is already present (orange) and can be removed or the dependency cannot be added (red), meaning it would be invalid or reversed in terms of time. Release your finger to complete the action of adding or removing a dependency.