Empty line histogram with single-item facet result

[ralphm]

For the histogram panel, if a query is such that the results would only cover
one point with the given interval, and the panel is configured to
show lines, not points or bars, the graph is entirely empty.

An example of a query response:

{
  "took" : 310,
  "timed_out" : false,
  "_shards" : {
    "total" : 10,
    "successful" : 10,
    "failed" : 0
  },
  "hits" : {
    "total" : 84974365,
    "max_score" : 1.0,
    "hits" : [ ]
  },
  "facets" : {
    "1" : {
      "_type" : "date_histogram",
      "entries" : [ {
        "time" : 1374773400000,
        "count" : 2
      } ]
    }
  }
}

[rashidkpc]

This was fixed with the addition of zero filling