ArcGIS Dashboard, Serial Chart Element - Updating the "Value Point" to a Different Shape via ArcGIS Online Assistant

[STan_RWC]

Specs: ArcGIS Online, ArcGIS Dashboards

I would like to know if there is the ability to change the bullet to anything but a circle for the Serial Chart element (aka line graphs). I experimented with the .json export from ArcGIS Online Assistant, and noticed that the "bullet:" can be changed. I experimented with "square" and "diamond"; both of those work. However, I would like to know if there are any other shapes that I can use. For example, I tried "triangle" but that didn't work. 

I was wondering if there is any documentation on the library or class that "bullet:" references. I couldn't find anything on ArcGIS Dashboards documentation published/available online.