Just checked out the Master repo to find this new gem.
It's just awesome. I've been having a ton of fun doodling space ships. When the lines, curves, and ellipses branch gets merged, it will really be something to behold. MyPaint has become the most artist friendly program I know.
A few requests, if I may.
Can we set an axis to draw upon? Ideally horizontal or vertical, but being able to set an arbitrary one at any angle could be interesting. I suppose this could just be done by rotating the canvas. I'm not sure what would be the best way to handle it.
Also, a way to set a position on the canvas in which to use the tool would be helpful. To go along with that, maybe a visual indicator or marker to show where the axis is would be nice. It's a bit awkward at first to start drawing and having to guess where the mirror axis is.
Again, thanks so much for implementing this. Even if these requests go ignored, just at it's current functionality, it's brilliant!