Haha, exactly, Pilou! You can just rotate all elements. For clothing patterns, your drawing itself provides the coordinate system, leaving you totally open to rotate all as you please. You can always select everything and rotate it. No need to rotate a canvas.
Circles are closed loops, which must not be trimmed. Arcs are for that and you can make an arc out of a circle by subdividing it, if you really had to. If you knew you wanted an arc, you'd really just make one, though. Arcs are evil, though. In my eyes they are the worst of all geometries, really. While they can do very little, they demand a great deal of consideration. Unlike Beziers, which are able to do virtually everything and they are super easy to deal with.
Tools like inkscape do naughty things, actually faking arcs with bezier curves! I know this, because I wrote a little SVG import, that left me quite surprised about this!
...
For accuracy I actually had to make arcs out of line segments. SVG arcs are notoriously imprecise for some reason?! When you give them an arc length (degree range), it does not virtually never go exactly to that length. Terrible. At least not in JavaFX.
...so I had to improvise, too.
Anyway, I'm SUPER thrilled you're experimenting around with it! That's awesome! But I can't tell you enough that it is truly meant and optimized for pattern making and does not really support free illustrations or any operations outside of the scope of clothing patterns. Like doing miniatures or the likes. But I may later try to make it a bit more ready for handling different scopes.