I actually did make a (less than spectacular) nonstatic Widget because I couldn't find the line chart option in the number widget. The one that I eventually found packed in SmartDashboard was way better in my opinion.
Here's a link to my code for it if anyone wants to look at how a custom Widget works
https://github.com/pordonj/frc1675-SmartDashboardPlot
Eventually the buffer size, max/min, etc. would have been configurable properties but I found the built-in one before I got that far I think.