What is the correct unit of SpeedIndex?

According to this https://sites.google.com/a/webpagetest.org/docs/usingwebpagetest/metrics/speedindex "The Speed Index is the average time at which visible parts of the page are displayed. It is expressed in milliseconds and dependent on size of the view port." Reading the explanation, I doubted that.
Essentially you're making an integral of the % visually incomplete. That means taking the area under the curve % v Time. Which to me suggest the metric has something like units of %/s.
Any thoughts on this?

Learn something new today..! if you will read the complete article of Speed Index, it's calculated in the millisecond(ms). if you will observe the area plot there, you have two fields Visibility Complete(which don't have any unit) w.r.t Time(ms) based on the formula(integration) & its mention in an article as well.
The Speed Index is the "area above the curve" calculated in ms
Speed Index will be directly proportional to time after integration when you will integrate for any visually complete equation(based on the plot) for the limit from 0 to end where "end"(Time)field after integration, Speed index will be proportional to time.