[SPARK-12304][STREAMING] Make Spark Streaming web UI display more fri…

…endly Receiver graphs

Currently, the Spark Streaming web UI uses the same maxY when displays 'Input Rate Times& Histograms' and 'Per-Receiver Times& Histograms'.

This may lead to somewhat un-friendly graphs: once we have tens of Receivers or more, every 'Per-Receiver Times' line almost hits the ground.

This issue proposes to calculate a new maxY against the original one, which is shared among all the `Per-Receiver Times& Histograms' graphs.

Before:
![before-5](https://cloud.githubusercontent.com/assets/15843379/11761362/d790c356-a0fa-11e5-860e-4b834603de1d.png)

After:
![after-5](https://cloud.githubusercontent.com/assets/15843379/11761361/cfabf692-a0fa-11e5-97d0-4ad124aaca2a.png)

Author: proflin <proflin.me@gmail.com>

Closes #10318 from proflin/SPARK-12304.

streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala

@@ -392,9 +392,15 @@ private[ui] class StreamingPage(parent: StreamingTab)
       maxX: Long,
       minY: Double,
       maxY: Double): Seq[Node] = {
+    val maxYCalculated = listener.receivedEventRateWithBatchTime.values
+      .flatMap { case streamAndRates => streamAndRates.map { case (_, eventRate) => eventRate } }
+      .reduceOption[Double](math.max)
+      .map(_.ceil.toLong)
+      .getOrElse(0L)
+
     val content = listener.receivedEventRateWithBatchTime.toList.sortBy(_._1).map {
       case (streamId, eventRates) =>
-        generateInputDStreamRow(jsCollector, streamId, eventRates, minX, maxX, minY, maxY)
+        generateInputDStreamRow(jsCollector, streamId, eventRates, minX, maxX, minY, maxYCalculated)
     }.foldLeft[Seq[Node]](Nil)(_ ++ _)
 
     // scalastyle:off