adds cx and cy for scale function

src/scale.js

@@ -1,19 +1,32 @@
 import { isUndefined } from './utils'
+import { translate } from './translate'
+import { transform } from './transform'
 
 /**
  * Calculate a scaling matrix
  * @param sx {number} Scaling on axis x
  * @param [sy = sx] {number} Scaling on axis y (default sx)
  * @returns {Matrix} Affine Matrix
  */
-export function scale (sx, sy = undefined) {
+export function scale (sx, sy = undefined, cx = undefined, cy = undefined) {
   if (isUndefined(sy)) sy = sx
-  return {
+
+  const scaleMatrix = {
     a: sx,
     c: 0,
     e: 0,
     b: 0,
     d: sy,
     f: 0
   }
+
+  if (isUndefined(cx) || isUndefined(cy)) {
+    return scaleMatrix
+  }
+
+  return transform([
+    translate(cx, cy),
+    scaleMatrix,
+    translate(-cx, -cy)
+  ])
 }

test/scale.spec.js

@@ -1,5 +1,5 @@
 /* global describe, it, expect */
-import { applyToPoint } from '../src/applyToPoint'
+import { applyToPoint, applyToPoints } from '../src/applyToPoint'
 import { scale } from '../src/scale'
 
 describe('scale', () => {
@@ -32,4 +32,34 @@ describe('scale', () => {
     expect(applyToPoint(m, { x: 50, y: 80 })).toEqual({ x: 1000, y: 1600 })
     expect(applyToPoint(scale(20, undefined), { x: 50, y: 80 })).toEqual({ x: 1000, y: 1600 })
   })
+
+  it('should scale points about specified origin', () => {
+    /*
+    <rect x="-50" y="-50" width="200" height="200" fill="green" />
+    <rect x="0" y="0" width="100" height="100" fill="black" />
+    <circle cx="50" cy="50" r="10" fill="red" />
+   */
+    const m = scale(2, 2, 50, 50)
+    expect(m).toEqual({
+      a: 2,
+      c: 0,
+      e: -50,
+      b: 0,
+      d: 2,
+      f: -50
+    })
+
+    const points = [
+      { x: 0, y: 0 },
+      { x: 100, y: 0 },
+      { x: 0, y: 100 },
+      { x: 100, y: 100 }
+    ]
+    expect(applyToPoints(m, points)).toEqual([
+      { x: -50, y: -50 },
+      { x: 150, y: -50 },
+      { x: -50, y: 150 },
+      { x: 150, y: 150 }
+    ])
+  })
 })