Improve accessibility of roadmaps with no start or end date

WCAG 1.4.1 Use of Color requirements state:

1.4.1 Use of Color: Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element. (Level A)

The opacity on the gradient on the roadmap gets very low and is hard to see, especially when hovering on a line item. How might we display lack of start or end date in a more accessible way that is easy for all users to discern?