Fix Timeline Example about cedar HOT 5 CLOSED

poke

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So - the problem is that in the time.json, we were using the data[0].url property to append in an orderByField into the query.

The #31 PR changed things so that the data is always fetched by Cedar and always passed to Vega as an array. Thus, this url hack to apply a sort can't work.

Since sorting is a "first class" problem, we should have a consistent means to address it.

Simplest Option

Allow the developer to specify a sort and attach that to the mappings. This string would be formatted exactly how the REST api expects ordreByFields to be, and would simply be merged into the query string.

  var dataset = {
    "url":"http://services.arcgis.com/bkrWlSKcjUDFDtgw/arcgis/rest/services/It's_a_Tornado_Map/FeatureServer/0",
    "mappings":{
      "time": {"field":"Date"},
      "value": {"field":"Injuries"},
      "sort":"Date DESC"
    }
  };
  chart.dataset = dataset;
  //or...
  chart.dataset.mappings.sort = "Date ASC";

This is nice because the sort could be applied to any field in the service.

Structured Option

Add a optional sort property to the mappings hash. It could have values of "ASC" or "DESC" which are the values associated with orderByFields param in the REST API

var dataset = {
    "url":"http://services.arcgis.com/bkrWlSKcjUDFDtgw/arcgis/rest/services/It's_a_Tornado_Map/FeatureServer/0",
    "mappings":{
      "time": {"field":"Date", "sort":"ASC"},
      "value": {"field":"Injuries"}
    }
  };

The limitation with that is that you could only sort by one of the mapped fields.

Conclusion

I built out support for both, but I feel that the first is a better option. This is because with sort, the order in which the sort fields are put into the orderByFields string impacts the output. Putting more than one sort into the mappings, will result in non-deterministic (or at least non-obvious) ordering of the orderByFields parameter. Explicitly setting the mappings.sort= 'FieldA ASC, FieldB DESC' affords complete control with low "surprise" factor. PRing in a moment

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PR for this is in #70

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LGTM http://esridc.github.io/cedar/examples/time.html. Closing.

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