Improve the positioning of labels aroud the ruler graphic

src/geometry.js

@@ -0,0 +1,21 @@
+export class Line {
+	constructor(m, b) {
+		this.m = m;
+		this.b = b;
+	}
+
+	static fromPoints(p1, p2) {
+		// Bring line into y=mx+b form
+		const m = (p1.y - p2.y) / (p1.x - p2.x);
+		const b = p1.y - m * p1.x;
+		return new Line(m, b);
+	}
+
+	get isVertical() {
+		return !isFinite(this.m);
+	}
+
+	calcY(x) {
+		return this.m * x + this.b;
+	}
+}