In the Burn-Down chart the vertical axis (remaining work) shows the amount of work (which is a sum of all the estimates for each item in the Product Backlog) and the horizontal axis shows the amount of time passed from the beginning of the project or the number of Sprints passed. We usually add another line to present the uniform distribution of the volume of the work across the initially estimated number of sprints. In the Burn-Up chart the vertical axis represents the amount of work and is measured in units or story points and the horizontal axis represents time usually measured in days. Source: https://www.artikelschreiber.com/en/.