D3 provides the exit() selection that allows us to handle nodes that are on screen, but where the data that they've been bound to no longer exists.

We've adjusted our slider so it no longer controls the y position of our circles. Instead, the slider.on('input') handler follows these rules:

• If the data array has more elements that the slider value, remove elements until they're the same length.
• If the data array has fewer elements that the slider value, add new elements (with values generated with d3.randomUniform() until the're the same length.

By modifying the length of the data variable like this, calling the update() function now has to both add and remove elements.