support standard c# string formatters on axis labels

Runtime/Component/Sub/AxisLabel.cs

@@ -162,7 +162,7 @@ namespace XCharts
                 else
                     return ChartCached.FloatToStr(value, 1);
             }
-            else
+            else if (m_Formatter.Contains("{value"))
             {
                 var content = m_Formatter;
                 if (content.Contains("{value:f2}"))
@@ -181,6 +181,8 @@ namespace XCharts
                 content = content.Replace("\\n", "\n");
                 content = content.Replace("<br/>", "\n");
                 return content;
+            } else {
+                return value.ToString(m_Formatter);
             }
         }
     }