Transformations

c.translate(0, 0)
c.rotate(Math.PI*2/5)
c.scale(1.0, 1.0)

To rotate along origin:

c.translate(ox, oy)
c.rotate(theta)
c.translate(-ox, -oy)

To scale along origin:

c.translate(-ox*x, -oy*y)
c.scale(x, y)
c.translate(ox/x, oy/y)

See MDN: Transformations.