Explaining Mike Bostock's d3.js dragmove function

Question

I\'m currently trying to create draggable behavior in my d3 script. I\'m studying Mike Bostock\'s example here: http://bl.ocks.org/mbostock/1557377

This is the function

Answer 1

It's being used to limit the drag range of each circle.

x is being limited to the range [radius, width-radius]

y is being limited to the range [radius, height-radius]

These are all inside an appropriately transformed svg g element so that x and y are relative to each cell. Hence the edges of the circles will cross the cell boundary when dragged.