make AAXAxis open

charts/src/main/java/com/github/aachartmodel/aainfographics/aaoptionsmodel/AAXAxis.kt

@@ -9,7 +9,7 @@
 package com.github.aachartmodel.aainfographics.aaoptionsmodel
 
 
-class AAXAxis: AAAxis() {
+open class AAXAxis: AAAxis() {
     fun allowDecimals(prop: Boolean?): AAXAxis {
         allowDecimals = prop
         return this
@@ -219,4 +219,4 @@ class AAXAxis: AAAxis() {
         tickPosition = prop
         return this
     }
-}
+}