Added ColorMap, and some popular maps from matplotlib (which their license appears to allow - "derivative works" only need to mention how they changed matplotlib, which I did)

Author: karwa

Sources/SwiftPlot/ColorMap.swift

@@ -0,0 +1,39 @@
+
+// This struct is a namespace for static functions and properties which return known color maps.
+// All of these functions and properties also _return_ a `ColorMap`,
+// instances of which are used as an interface type
+// rather than existentials of `ColorMapProtocol` directly
+// (again, so users can have access to convenient, known color maps).
+
+public struct ColorMap: ColorMapProtocol {
+  var base: ColorMapProtocol
+  
+  public init(_ base: ColorMapProtocol) {
+    if let existing = base as? ColorMap { self = existing }
+    else { self.base = base }
+  }
+  
+  public func colorForOffset(_ offset: Float) -> Color {
+    return base.colorForOffset(offset)
+  }
+}
+
+public protocol ColorMapProtocol {
+  func colorForOffset(_ offset: Float) -> Color
+}
+
+// Linear Gradients.
+
+struct LinearGradient: ColorMapProtocol {
+  var start: Color
+  var end: Color
+  func colorForOffset(_ offset: Float) -> Color {
+    return lerp(startColor: start, endColor: end, offset)
+  }
+}
+
+extension ColorMap {
+  public static func linear(_ start: Color, _ end: Color) -> ColorMap {
+    return ColorMap(LinearGradient(start: start, end: end))
+  }
+}