Optimize AAStyle padding set function

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

@@ -101,8 +101,10 @@ class AAStyle {
         return this
     }
 
-    fun padding(prop: String?): AAStyle {
-        padding = prop
+    fun padding(prop: Number?): AAStyle {
+        prop?.let {
+            padding = "${prop}px"
+        }
         return this
     }
 

sample/src/main/java/com/github/aachartmodel/aainfographics/demo/chartcomposer/ChartOptionsComposer.kt

@@ -189,7 +189,7 @@ object ChartOptionsComposer {
             .border("2px solid red")
             .opacity(1.0)
             .fontWeight(AAChartFontWeightType.Bold)
-            .padding("6px")
+            .padding(6)
             .fontSize(16)
 
         val aaPlotLinesElementsArr = arrayOf(