Don't use empty to calculate bounding box

The `BoundingBox` from Doodle does not the semantics we want for
calculating the bounding box of data. `BoundingBox` must always contain
the origin, which is not the case for data. `BoundingBox.empty` is a
point at the origin. Using it means we include the origin when
calculating the bounding box of data, which is incorrect.

There must be a least one `Layer` in a `Plot`, so use a `NonEmptySeq` to
represent this.

core/shared/src/main/scala/chartreuse/Data.scala

@@ -27,7 +27,24 @@ enum Data[+A] {
   case FromTraverse[F[_], A](data: F[A], traverse: Traverse[F]) extends Data[A]
 
   def boundingBox(toPoint: A => Point): BoundingBox =
-    foldLeft(BoundingBox.empty)((bb, a) => bb.enclose(toPoint(a)))
+    this match {
+      case FromIterable(data) =>
+        if data.isEmpty then BoundingBox.empty
+        else {
+          val pt = toPoint(data.head)
+          val bb = BoundingBox(pt.x, pt.y, pt.x, pt.y)
+          data.tail.foldLeft(bb) { (bb, a) => bb.enclose(toPoint(a)) }
+        }
+      case FromTraverse(data, traverse) =>
+        traverse.reduceLeftToOption(data) { (a: A) =>
+          val pt = toPoint(a)
+          BoundingBox(pt.x, pt.y, pt.x, pt.y)
+        }((bb, a) => bb.enclose(toPoint(a))) match {
+          case Some(value) => value
+          case None => BoundingBox.empty
+        }
+
+    }
 
   def foldLeft[B](z: B)(f: (B, A) => B): B =
     this match {

core/shared/src/main/scala/chartreuse/Plot.scala

@@ -17,6 +17,7 @@
 package chartreuse
 
 import cats.Id
+import cats.data.NonEmptySeq
 import chartreuse.component.Axis.*
 import chartreuse.component.*
 import chartreuse.theme.PlotTheme
@@ -30,7 +31,7 @@ import Plot.PlotAlg
   * grid.
   */
 final case class Plot[-Alg <: Algebra](
-    layers: List[Layer[?, Alg]],
+    layers: NonEmptySeq[Layer[?, Alg]],
     plotTitle: String = "Plot Title",
     xTitle: String = "X data",
     yTitle: String = "Y data",
@@ -43,7 +44,7 @@ final case class Plot[-Alg <: Algebra](
     annotations: List[Annotation] = List.empty[Annotation]
 ) {
   def addLayer[Alg2 <: Algebra](layer: Layer[?, Alg2]): Plot[Alg & Alg2] = {
-    copy(layers = layer :: layers)
+    copy(layers = layer +: layers)
   }
 
   def addAnnotation(annotation: Annotation): Plot[Alg] = {
@@ -89,9 +90,7 @@ final case class Plot[-Alg <: Algebra](
       height: Int,
       theme: PlotTheme[Id] = PlotTheme.default
   ): Picture[Alg & PlotAlg, Unit] = {
-    val dataBoundingBox = layers.foldLeft(BoundingBox.empty) { (bb, layer) =>
-      bb.on(layer.boundingBox)
-    }
+    val dataBoundingBox = layers.map(_.boundingBox).reduceLeft(_.on(_))
 
     val dataMinX = dataBoundingBox.left
     val dataMaxX = dataBoundingBox.right
@@ -216,5 +215,5 @@ object Plot {
 
   /** Utility constructor to create a `Plot` from a single layer. */
   def apply[Alg <: Algebra](layer: Layer[?, Alg]): Plot[Alg] =
-    Plot(layers = List(layer))
+    Plot(layers = NonEmptySeq.one(layer))
 }

core/shared/src/main/scala/chartreuse/component/Legend.scala

@@ -17,6 +17,7 @@
 package chartreuse.component
 
 import cats.Id
+import cats.data.NonEmptySeq
 import chartreuse.Layer
 import chartreuse.Plot.PlotAlg
 import chartreuse.theme.PlotTheme
@@ -25,15 +26,15 @@ import doodle.core.*
 import doodle.syntax.all.*
 
 final case class Legend[-Alg <: Algebra](
-    layers: Seq[Layer[?, Alg]],
+    layers: NonEmptySeq[Layer[?, Alg]],
     theme: PlotTheme[Id]
 ) {
   def build(x: Double, y: Double): Picture[Alg & PlotAlg, Unit] = {
     val circleRadius = 8
     val legendMargin = 6
 
     val legendContent =
-      (layers
+      (layers.iterator
         .zip(theme.layerThemesIterator))
         .foldLeft(empty[Alg & PlotAlg])((content, layerAndTheme) => {
           // This code is not ideal, because we're recreating the themed value here,

examples/src/main/scala/chartreuse/examples/BahamasPopulation.scala

@@ -16,6 +16,7 @@
 
 package chartreuse.examples
 
+import cats.data.NonEmptySeq
 import cats.effect.unsafe.implicits.global
 import chartreuse.*
 import chartreuse.layout.*
@@ -303,7 +304,7 @@ object BahamasPopulation {
 
   val plot =
     Plot(
-      List(
+      NonEmptySeq.of(
         line.toLayer(population).withLabel("Line"),
         curve.toLayer(population).withLabel("Curve")
       )

examples/src/main/scala/chartreuse/examples/PlotExample.scala

@@ -16,6 +16,7 @@
 
 package chartreuse.examples
 
+import cats.data.NonEmptySeq
 import cats.effect.unsafe.implicits.global
 import chartreuse.*
 import chartreuse.layout.Scatter
@@ -41,7 +42,7 @@ object PlotExample {
   }
 
   val plot: Plot[Basic] = Plot(
-    List.fill(5)(randomLayer),
+    NonEmptySeq.fromSeqUnsafe(List.fill(5)(randomLayer)),
     grid = true
   )
 

examples/src/main/scala/chartreuse/examples/TemperatureAnomaly.scala

@@ -16,6 +16,7 @@
 
 package chartreuse.examples
 
+import cats.data.NonEmptySeq
 import cats.effect.unsafe.implicits.global
 import chartreuse.*
 import chartreuse.layout.*
@@ -36,30 +37,33 @@ object TemperatureAnomaly {
     data.groupBy(_.year).filter((year, _) => year < 2023)
 
   val layers =
-    dataByYear
-      .map((year, records) =>
-        Line
-          .default[Record]
-          .forThemeable(themeable =>
-            // Highlight the most recent years by themeing them. Other years become grey
-            if year < 2013 then
-              themeable.withStrokeColor(Themeable.Override(Some(Color.grey)))
-            else themeable
-          )
-          .toLayer(records.sortBy(_.month))(record =>
-            Point(record.month, record.anomaly)
-          )
-          .withLabel(year.toString)
-      )
-      .toList
-      .sortBy(_.label)
+    NonEmptySeq.fromSeqUnsafe(
+      dataByYear
+        .map((year, records) =>
+          Line
+            .default[Record]
+            .forThemeable(themeable =>
+              // Highlight the most recent years by themeing them. Other years become grey
+              if year < 2013 then
+                themeable.withStrokeColor(Themeable.Override(Some(Color.grey)))
+              else themeable
+            )
+            .toLayer(records.sortBy(_.month))(record =>
+              Point(record.month, record.anomaly)
+            )
+            .withLabel(year.toString)
+        )
+        .toSeq
+        .sortBy(_.label)
+    )
 
-  val plot = Plot(layers.toList)
+  val plot = Plot(layers)
     .withPlotTitle(
       "Global Average Temperature Anomaly (2022-2013 Highlighted)"
     )
     .withYTitle("°C anomaly from 1961-1990")
     .withXTitle("Month")
+    .withXTicks(MajorTickLayout.Manual(Seq(1,2,3,4,5,6,7,8,9,10,11,12)))
     .withLegend(false)
 
   @JSExport