Graph

{

 "width": 400,
 "height": 200,
 "padding": {"top": 10, "left": 30, "bottom": 20, "right": 10},

 "data": [
   {
     "name": "table",
     "values": [
       {"category":"A", "amount":28},
       {"category":"B", "amount":55},
       {"category":"C", "amount":43},
       {"category":"D", "amount":91},
       {"category":"E", "amount":81},
       {"category":"F", "amount":53},
       {"category":"G", "amount":19},
       {"category":"H", "amount":87},
       {"category":"I", "amount":52}
     ]
   }
 ],

 "signals": [
   {
     "name": "tooltip",
     "init": {},
     "streams": [
       {"type": "rect:mouseover", "expr": "datum"},
       {"type": "rect:mouseout", "expr": "{}"}
     ]
   }
 ],

 "predicates": [
   {
     "name": "tooltip", "type": "==", 
     "operands": [{"signal": "tooltip._id"}, {"arg": "id"}]
   }
 ],

 "scales": [
   { "name": "xscale", "type": "ordinal", "range": "width",
     "domain": {"data": "table", "field": "category"} },
   { "name": "yscale", "range": "height", "nice": true,
     "domain": {"data": "table", "field": "amount"} }
 ],

 "axes": [
   { "type": "x", "scale": "xscale" },
   { "type": "y", "scale": "yscale" }
 ],

 "marks": [
   {
     "type": "rect",
     "from": {"data":"table"},
     "properties": {
       "enter": {
         "x": {"scale": "xscale", "field": "category"},
         "width": {"scale": "xscale", "band": true, "offset": -1},
         "y": {"scale": "yscale", "field": "amount"},
         "y2": {"scale": "yscale", "value":0}
       },
       "update": { "fill": {"value": "steelblue"} },
       "hover": { "fill": {"value": "red"} }
     }
   },
   {
     "type": "text",
     "properties": {
       "enter": {
         "align": {"value": "center"},
         "fill": {"value": "#333"}
       },
       "update": {
         "x": {"scale": "xscale", "signal": "tooltip.category"},
         "dx": {"scale": "xscale", "band": true, "mult": 0.5},
         "y": {"scale": "yscale", "signal": "tooltip.amount", "offset": -5},
         "text": {"signal": "tooltip.amount"},
         "fillOpacity": {
           "rule": [
             {
               "predicate": {"name": "tooltip", "id": {"value": null}},
               "value": 0
             },
             {"value": 1}
           ]
         }
       }
     }
   }
 ]

}