d3.timer(callback[, delay[, time]])
Schedules a new timer, invoking the specified callback repeatedly until the timer is stopped. An optional numeric delay in milliseconds may be specified to invoke the given callback after a delay; if delay is not specified, it defaults to zero. The delay is relative to the specified time in milliseconds; if time is not specified, it defaults to now.
The callback is passed the (apparent) elapsed time since the timer became active. For example:
var t = d3.tim