summarize(): in !alignToFrom case, return output in canonical format

See #1811 for more details
Before, it was common for output to contain an extraneous point that
lies before 'from' (and wouldn't be rendered by Grafana in graph
panels), which made the output invalid.

This would result in trouble when

* combining such a series with another series
  (#1811)
* or when needing normalization
  (#1845)

This effectively slightly changes the output format, but is more robust.

expr/func_summarize.go

@@ -95,6 +95,8 @@ func summarizeValues(serie models.Series, aggFunc batch.AggFunc, interval, start
 
 	numPoints := len(serie.Datapoints)
 
+	// graphite-compatible bit
+
 	for ts, i := start, 0; i < numPoints && ts < end; ts += interval {
 		s := i
 		for ; i < numPoints && serie.Datapoints[i].Ts < ts+interval; i++ {
@@ -111,5 +113,11 @@ func summarizeValues(serie models.Series, aggFunc batch.AggFunc, interval, start
 		out = append(out, aggPoint)
 	}
 
+	// MT specific bit: if !s.alignToFrom we want the output to be canonical
+	// only thing needed is strip out the first point if its TS < from
+	if len(out) != 0 && out[0].Ts < serie.QueryFrom {
+		out = out[1:]
+	}
+
 	return out
 }