can just supply inverted ranges.

xaxis.go

@@ -1,9 +1,6 @@
 package chart
 
-import (
-	"math"
-	"sort"
-)
+import "math"
 
 // XAxis represents the horizontal axis.
 type XAxis struct {
@@ -71,7 +68,6 @@ func (xa XAxis) GetGridLines(ticks []Tick) []GridLine {
 // Measure returns the bounds of the axis.
 func (xa XAxis) Measure(r Renderer, canvasBox Box, ra Range, defaults Style, ticks []Tick) Box {
 	tickStyle := xa.TickStyle.InheritFrom(xa.Style.InheritFrom(defaults))
-	sort.Sort(Ticks(ticks))
 
 	tp := xa.GetTickPosition()
 
@@ -124,8 +120,6 @@ func (xa XAxis) Render(r Renderer, canvasBox Box, ra Range, defaults Style, tick
 	r.LineTo(canvasBox.Right, canvasBox.Bottom)
 	r.Stroke()
 
-	sort.Sort(Ticks(ticks))
-
 	tp := xa.GetTickPosition()
 
 	var tx, ty int