Added a heatmap function for all Collections. It requires an O(n) scan to determine the row locations, but doesn't require copying in to a RAC (which would also be O(n), *and* consume memory to boot).

Allows direct heat-mapping of Strings (is that useful?). It would be nice if all charts did this and namespaced it under ".plots" or something (see ".lazy" in the Swift stdlib).

Author: karwa

Sources/SwiftPlot/Heatmap.swift

@@ -165,6 +165,31 @@ extension Sequence where Element: Sequence {
   }
 }
 
+extension Collection {
+  
+  /// Returns a heatmap of this collection's values, generated by slicing rows with the given width.
+  /// - parameters:
+  ///   - width:        The width of the heatmap to generate. Must be greater than 0.
+  ///   - interpolator: A function or `KeyPath` which maps values to a continuum between 0 and 1.
+  /// - returns: A heatmap plot of the collection's values.
+  /// - complexity: O(n). Consider though, that rendering a heatmap or copying to a `RamdomAccessCollection`
+  ///               is also at least O(n), and this does not copy the data.
+  public func heatmap(width: Int, interpolator: Interpolator<Element>) -> Heatmap<[SubSequence]> {
+    precondition(width > 0, "Cannot build a histogram with zero or negative width")
+    var rows = [SubSequence]()
+    var rowStart = startIndex
+    while rowStart != endIndex {
+      guard let rowEnd = index(rowStart, offsetBy: width, limitedBy: endIndex) else {
+        rows.append(self[rowStart..<endIndex])
+        break
+      }
+      rows.append(self[rowStart..<rowEnd])
+      rowStart = rowEnd
+    }
+    return rows.heatmap(interpolator: interpolator)
+  }
+}
+
 extension RandomAccessCollection {
 
   /// Returns a heatmap of this collection's values, generated by slicing rows with the given width.

Tests/SwiftPlotTests/Heatmap/heatmap.swift

@@ -32,9 +32,10 @@ final class HeatmapTests: SwiftPlotTestCase {
     for _ in 0..<10_000 { d.append(.random(in: 0..<255)) }
 //    var hm3 = d.heatmap(width: 100, interpolator: .linear)
 
-    var hm3 = Array("THIS IS SWIFPLOT!!!! Woo let's see what this looks like :)")
-      .heatmap(height: 1, interpolator: .linearByKeyPath(\.asciiValue!))
+    var hm3 = "THIS IS SWIFPLOT!!!! Woo let's see what this looks like :)"
+      .heatmap(width: 5, interpolator: .linearByKeyPath(\.asciiValue!))
 //    var hm3 = Array(stride(from: Float(0), to: 1, by: 0.001)).heatmap(width: 10, interpolator: .linear)
+//    var hm3 = "😘 you too".utf8.heatmap(width: 5, interpolator: .linear)
     hm3.colorMap = .viridis