I have a series of drill-through column charts, where you click on one column and it passes a parameter through to the next chart and so on, giving the appearance of drill-down.

My graphs consist of 3 types of labor, and have three colors on the main chart. When I drill down to the next chart, some of the categories do not have all three types of labor that the main one has. So the first color in the palette is assigned to the series, even though it was the second color on the previous chart. I’d like to avoid this, if possible.

So a data value is green on the first chart (2nd in the color order) and yellow on the next chart (1st in the color order). I needed to make the graphs “remember” the total number of series groups that were in the first chart.

I was able to solve this because I was using a custom color palette, implemented as a hash table. I basically serialized this information and passed it to a hidden parameter on the subreport and then reinflated the data structure.

It’s not perfect, but it works for now.

' Define some globals, including the color palette
Private colorPalette As String() = _
    {"#FFF8A3", "#A9CC8F", "#B2C8D9", "#BEA37A", "#F3AA79", "#B5B5A9", "#E6A5A4", _
     "#F8D753", "#5C9746", "#3E75A7", "#7A653E", "#E1662A", "#74796F", "#C4384F", _
     "#F0B400", "#1E6C0B", "#00488C", "#332600", "#D84000", "#434C43", "#B30023"}
     ' color palette pulled from SAP guidelines
     ' http://www.sapdesignguild.org/resources/diagram_guidelines/color_palettes.html
Private count As Integer = 0
Private colorMapping As New System.Collections.Hashtable()
' Create a custom color palette
Public Function GetColor(ByVal groupingValue As String) As String
    If colorMapping.ContainsKey(groupingValue) Then
        Return colorMapping(groupingValue)
    End If
    Dim c As String = colorPalette(count Mod colorPalette.Length)
    count = count + 1
    colorMapping.Add(groupingValue, c)
    Return c
End Function 

' In custom actions of the data value, set the results of this
' function to the mapping parameter in the next report
Public Function PassColorMapping() As String
    If colorMapping.Count = 0 Then
        Return Nothing
    End If
    Try
        ' convert the hashtable to an array so it can be serialized
        Dim objHash As Object()() = ToJaggedArray(colorMapping) 

        ' serialize the colorMapping variable
        Dim outStream As New System.IO.StringWriter()
        Dim s As New System.Xml.Serialization.XmlSerializer(GetType(Object()()))
        s.Serialize(outStream, objHash) 

        ' move on to the next report
        Return outStream.ToString()
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Function

I ran into an issue where I couldn’t find the equivalent of the onLoad event for the report. Since I wasn’t sure where to put this inflate code, I stuck it in the background color of the plot area. Hence I always return “WhiteSmoke”. I’ll change this if I can find the right place to put it.

' Call this function when the report loads to get the series groups
' that have already been loaded into the custom color palette
' Pass in the parameter used to store the color mapping
Public Function InflateParamMapping(ByVal paramMapping As Parameter) As String
    Try
        If paramMapping.Value Is Nothing Then
            Return "WhiteSmoke"
        ElseIf colorMapping.Count = 0 Then
            Dim pXmlized As String = paramMapping.Value
            ' deserialize the mapping parameter
            Dim s As New System.Xml.Serialization.XmlSerializer(GetType(Object()())) 

            ' get the jagged array and convert to hashtable
            Dim objHash As Object()() = DirectCast(s.Deserialize(New System.IO.StringReader(pXmlized)), Object()())
            ' stick the result in the global colorMapping hashtable
            colorMapping = ToHashTable(objHash)
            count = colorMapping.Count
        End If
    Catch ex As Exception
'       MsgBox(ex.Message)
    End Try
    Return "WhiteSmoke"
End Function

ToJaggedArray() and ToHashTable() are helper functions because a HashTable is not serializable since they implement an IDictionary. I was in a hurry so I just converted them to an array right quick. Code comes from the Collection Serialization in ASP.NET Web Services article written by Mark Richman. I converted the code from C# to VB.NET to use in the report.

Public Function ToJaggedArray(ByVal ht As System.Collections.HashTable) As Object()()
    Dim oo As Object()() = New Object(ht.Count - 1)() {}
    Dim i As Integer = 0
    For EAch key As Object in ht.Keys
        oo(i) = New Object() {key, ht(key)}
        i += 1
    Next
    Return oo
End Function 

Public Function ToHashTable(ByVal oo As Object()()) As System.Collections.HashTable
    Dim ht As New System.Collections.HashTable(oo.Length)
    For Each pair As Object() In oo
        Dim key As Object = pair(0)
        Dim value As Object = pair(1)
        ht(key) = value
    Next
    Return ht
End Function

Now in the report itself you need to do a couple things.

• Add a reference to System.Xml in Report Properties in both reports.
• In the Actions of your parent report, set the Parameter containing your data structure to =Code.PassColorMapping()
• In the Plot Area section of your report, put this expression for the background: =Code.InflateParamMapping(Parameters!colorMapping)
• And of course, in the Fill for your data Series Style on both charts put this expression: =Code.GetColor(Fields!Type.Value)

You can continue doing this for as many subreports as you want – I currently have 3 levels of drill-through and it works fine.