+/**
+ * The value of datefield will start at an even multiple of "step", i.e.
+ * if datefield=SS and step=5 then the first tick will be on a multiple of 5s.
+ *
+ * For granularities <= HOURLY, ticks are generated every `spacing` ms.
+ *
+ * At coarser granularities, ticks are generated by incrementing `datefield` by
+ * `step`. In this case, the `spacing` value is only used to estimate the
+ * number of ticks. It should roughly correspond to the spacing between
+ * adjacent ticks.
+ *
+ * @type {Array.<{datefield:number, step:number, spacing:number}>}
+ */