Thank you, I will file a bug report.
For anyone else that finds this, the scrollbar:
always shows on my Mac Mini (mouse and keyboard only)
does not show on my Macbook Air (trackpad)
automatically shows when I connect a bluetooth mouse to the Macbook Air, but still does not allow mouse interaction. When I turn off the bluetooth mouse, the scrollbar disappeared.
The trackpad allows for scrolling the chart with 2-finger swipe gestures, with or without the bluetooth mouse connected.