Support secondary axis for line chart

Author: Mischa Spelt

blazorbootstrap/Models/Charts/ChartDataset/LineChart/LineChartDataset.cs

@@ -329,7 +329,10 @@ public class LineChartDataset : ChartDataset<double?>
   /// Default value is 'first y axis'.
   /// </remarks>
   [JsonIgnore( Condition = JsonIgnoreCondition.WhenWritingNull )]
-  public string? YAxisID { get; set; }
+  public string? YAxisID => this.OnSecondaryYAxis ? "Y1" : null;
+
+  [JsonIgnore]
+  public bool OnSecondaryYAxis { get; set; }
 
   #endregion
 }