I've now fixed this in Rockpool Edge.
For your Joystick and Matrix project, you can create two rules in Edge. One for Joystick X and one for Joystick Y, then map these to Plot X and Ploy Y on the Matrix.
Rockpool is, intentionally, very limited so you may find you'll have to delve into Python to do some of the things you want. We're investigating ways to make this transition easier for people who aren't familiar with Python.
The flow is very much "Rockpool" -> "???" -> Python, at the moment. We originally intended for Scratch to fill that middle step, but Scratch really didn't lend itself well to the "plug and play" nature of Flotilla.
Your feedback is appreciated. Right now we're beginning our official "Month Of Flotilla," so it couldn't have come at a better time!