Vega time ticks precision

time-ticks.json

{
   "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
   "data": {
       "values": [
           {
               "date": "2018-05-01",
               "value": 1
           },
           {
               "date": "2018-05-02",
               "value": 2
           },
           {
               "date": "2018-05-03",
               "value": 3
           },
           {
               "date": "2018-05-04",
               "value": 2
           }
       ]
   },
   "layer": [
       {
           "mark": "line",
           "encoding": {
               "x": {
                   "field": "date",
                   "type": "temporal"
               },
               "y": {
                   "field": "value",
                   "type": "quantitative"
               }
           }
       },
       {
           "data": {
               "values": [
                   {
                       "date": "2018-05-03"
                   }
               ]
           },
           "mark": "rule",
           "encoding": {
               "x": {
                   "field": "date",
                   "type": "temporal",
                   "axis": {
                       "labelAngle": -90
                   }
               }
           }
       }
   ]
}