Hold primary button: Draw line.
Secondary or middle button: Toggle gravity point.
A key: Create gravity point in the starting point of the last stroke.
E key: Create gravity point in the end point of the last stroke.
D key: Delete all gravity points.

Hole radius: Radius of the area in the tablet or source screen that is mapped exactly to the gravity center.
Force radius: Radius of effect of the gravity point. Outside of this radius the gravity point has no effect.
If a point is under the influence of several gravity points, their effects are averaged.

Browser support:
Works best in a desktop browser with getCoalescedEvents() support (as of writing only Chrome 58+).
Touch input is supported, but it is not accurate because there is no API for coalesced touch events.
Does not play accurately with UI scaling as that would require browser support for fractional coordinates, which as of writing have no browser support (W3C issue).