chart: fix multi-Y limit inconsistency

atlas-chart/src/main/resources/reference.conf

@@ -86,7 +86,7 @@ atlas {
       max-height = 1000
       max-zoom = 2.0
       max-lines-in-legend = 50
-      max-yaxes = 4
+      max-yaxes = 5
     }
   }
 }

atlas-eval/src/main/scala/com/netflix/atlas/eval/graph/Grapher.scala

@@ -116,7 +116,7 @@ case class Grapher(settings: DefaultSettings) {
       .getOrElse(Features.STABLE)
 
     import com.netflix.atlas.chart.GraphConstants.*
-    val axes = (0 to MaxYAxis).map(i => i -> newAxis(params, i)).toMap
+    val axes = (0 until MaxYAxis).map(i => i -> newAxis(params, i)).toMap
 
     val vision = params.get("vision").map(v => VisionType.valueOf(v))
 
@@ -311,7 +311,7 @@ case class Grapher(settings: DefaultSettings) {
 
     val plots = plotExprs.toList.sortWith(_._1 < _._1).map {
       case (yaxis, exprs) =>
-        val axisCfg = config.flags.axes(yaxis)
+        val axisCfg = config.flags.getAxis(yaxis)
         val dfltStyle = if (axisCfg.stack) LineStyle.STACK else LineStyle.LINE
 
         val statFormatter = axisCfg.tickLabelMode match {

atlas-eval/src/main/scala/com/netflix/atlas/eval/graph/ImageFlags.scala

@@ -45,4 +45,11 @@ case class ImageFlags(
       case None       => settings.primaryPalette(theme)
     }
   }
+
+  def getAxis(index: Int): Axis = {
+    axes.get(index) match {
+      case Some(axis) => axis
+      case None       => throw new IllegalArgumentException(s"invalid axis: $index")
+    }
+  }
 }

atlas-eval/src/test/scala/com/netflix/atlas/eval/graph/GrapherSuite.scala

@@ -290,6 +290,25 @@ class GrapherSuite extends FunSuite {
       "&ylabel.0=Axis%200&ylabel.1=Axis%201"
   }
 
+  imageTest("multi-Y, 5 axes") {
+    "/api/v1/graph?e=2012-01-01T00:00&q=(,0,1,2,3,4,),(,:dup,:axis,),:each"
+  }
+
+  test("multi-Y, 6 axes") {
+    val uri = "/api/v1/graph?e=2012-01-01T00:00&q=(,0,1,2,3,4,6,),(,:dup,:axis,),:each"
+    intercept[IllegalArgumentException] {
+      grapher.evalAndRender(Uri(uri), db)
+    }
+  }
+
+  imageTest("multi-Y, 5 axis per line") {
+    "/api/v1/graph?e=2012-01-01T00:00&q=0,1,2,3,4&axis_per_line=1"
+  }
+
+  imageTest("multi-Y, 6 axis per line") {
+    "/api/v1/graph?e=2012-01-01T00:00&q=0,1,2,3,4,6&axis_per_line=1"
+  }
+
   imageTest("significant time boundaries and tz=US/Pacific") {
     "/api/v1/graph?q=name,sps,:eq,:sum&s=e-2d&e=2015-06-17T13:13&no_legend=1&tz=US/Pacific"
   }