SSRS: Change the legend shape
I was able to do this with a matrix. First you have to insert a rectangle in the cell you want to use for the legend. Then the assign the background color of the rectangle to the column in an expression e.g. =Fields!ColorHex.Value. Next insert the circle image over top of the rectangle.
You should be able to use the same dataset for the matrix (legend) as you do for the chart. You'll need to add a column for the ColorHex values to your dataset. Then you can also reference the same column in the chart for the colors.
Report Preview:
https://i.stack.imgur.com/Vs2Ab.png
Circle .png file with transparency:
I created the image using GIMP (GNU Image Manipulation Program)
https://i.stack.imgur.com/NXy8j.png
Example SQL for color hex codes
SELECT tbl.* FROM (VALUES
( '#90EE90', 'Due next month')
, ( '#FAFAD2', 'Due this month')
, ( '#FFB6C1', 'Overdue')
) tbl ([ColorHex], [GroupType])
Example XML for the .rdl report:
<?xml version="1.0" encoding="utf-8"?>
<Report MustUnderstand="df" xmlns="http://schemas.microsoft.com/sqlserver/reporting/2016/01/reportdefinition" xmlns:rd="http://schemas.microsoft.com/SQLServer/reporting/reportdesigner" xmlns:df="http://schemas.microsoft.com/sqlserver/reporting/2016/01/reportdefinition/defaultfontfamily">
<df:DefaultFontFamily>Segoe UI</df:DefaultFontFamily>
<AutoRefresh>0</AutoRefresh>
<DataSources>
<DataSource Name="YourDataSource">
<DataSourceReference>YourDataSource</DataSourceReference>
<rd:SecurityType>None</rd:SecurityType>
<rd:DataSourceID>68eb8e17-19b1-4ab3-8315-91c1cb31c63a</rd:DataSourceID>
</DataSource>
</DataSources>
<DataSets>
<DataSet Name="DataSet1">
<Query>
<DataSourceName>YourDataSource</DataSourceName>
<CommandText>SELECT tbl.* FROM (VALUES
( '#90EE90', 'Due next month')
, ( '#FAFAD2', 'Due this month')
, ( '#FFB6C1', 'Overdue')
) tbl ([ColorHex], [GroupType]) </CommandText>
</Query>
<Fields>
<Field Name="ColorHex">
<DataField>ColorHex</DataField>
<rd:TypeName>System.String</rd:TypeName>
</Field>
<Field Name="GroupType">
<DataField>GroupType</DataField>
<rd:TypeName>System.String</rd:TypeName>
</Field>
</Fields>
</DataSet>
</DataSets>
<ReportSections>
<ReportSection>
<Body>
<ReportItems>
<Tablix Name="Tablix3">
<TablixBody>
<TablixColumns>
<TablixColumn>
<Width>0.9125cm</Width>
</TablixColumn>
<TablixColumn>
<Width>3.21438cm</Width>
</TablixColumn>
</TablixColumns>
<TablixRows>
<TablixRow>
<Height>0.89146cm</Height>
<TablixCells>
<TablixCell>
<CellContents>
<Rectangle Name="Rectangle3">
<ReportItems>
<Image Name="Image3">
<Source>Embedded</Source>
<Value>legend_template</Value>
<Sizing>FitProportional</Sizing>
<Height>0.89146cm</Height>
<Width>0.9125cm</Width>
<Style>
<Border>
<Style>None</Style>
</Border>
</Style>
</Image>
</ReportItems>
<KeepTogether>true</KeepTogether>
<Style>
<Border>
<Style>None</Style>
</Border>
<RightBorder>
<Color>White</Color>
<Style>Solid</Style>
</RightBorder>
<BackgroundColor>=Fields!ColorHex.Value</BackgroundColor>
</Style>
</Rectangle>
</CellContents>
</TablixCell>
<TablixCell>
<CellContents>
<Textbox Name="GroupType1">
<CanGrow>true</CanGrow>
<KeepTogether>true</KeepTogether>
<Paragraphs>
<Paragraph>
<TextRuns>
<TextRun>
<Value>=Fields!GroupType.Value</Value>
<Style>
<FontFamily>Calibri</FontFamily>
</Style>
</TextRun>
</TextRuns>
<Style />
</Paragraph>
</Paragraphs>
<rd:DefaultName>GroupType1</rd:DefaultName>
<Style>
<Border>
<Color>LightGrey</Color>
<Style>None</Style>
</Border>
<VerticalAlign>Middle</VerticalAlign>
<PaddingLeft>2pt</PaddingLeft>
<PaddingRight>2pt</PaddingRight>
<PaddingTop>2pt</PaddingTop>
<PaddingBottom>2pt</PaddingBottom>
</Style>
</Textbox>
</CellContents>
</TablixCell>
</TablixCells>
</TablixRow>
</TablixRows>
</TablixBody>
<TablixColumnHierarchy>
<TablixMembers>
<TablixMember>
<Group Name="GroupType">
<GroupExpressions>
<GroupExpression>=Fields!GroupType.Value</GroupExpression>
</GroupExpressions>
</Group>
<SortExpressions>
<SortExpression>
<Value>=Fields!GroupType.Value</Value>
</SortExpression>
</SortExpressions>
<TablixMembers>
<TablixMember />
<TablixMember />
</TablixMembers>
</TablixMember>
</TablixMembers>
</TablixColumnHierarchy>
<TablixRowHierarchy>
<TablixMembers>
<TablixMember>
<Group Name="RowGroup">
<GroupExpressions>
<GroupExpression />
</GroupExpressions>
</Group>
<TablixHeader>
<Size>0.07938cm</Size>
<CellContents>
<Textbox Name="Textbox9">
<CanGrow>true</CanGrow>
<KeepTogether>true</KeepTogether>
<Paragraphs>
<Paragraph>
<TextRuns>
<TextRun>
<Value />
<Style />
</TextRun>
</TextRuns>
<Style />
</Paragraph>
</Paragraphs>
<rd:DefaultName>Textbox9</rd:DefaultName>
<Style>
<Border>
<Color>LightGrey</Color>
<Style>None</Style>
</Border>
<PaddingLeft>2pt</PaddingLeft>
<PaddingRight>2pt</PaddingRight>
<PaddingTop>2pt</PaddingTop>
<PaddingBottom>2pt</PaddingBottom>
</Style>
</Textbox>
</CellContents>
</TablixHeader>
</TablixMember>
</TablixMembers>
</TablixRowHierarchy>
<DataSetName>DataSet1</DataSetName>
<Height>0.89146cm</Height>
<Width>4.20625cm</Width>
<Style>
<Border>
<Style>None</Style>
</Border>
</Style>
</Tablix>
</ReportItems>
<Height>0.35097in</Height>
<Style />
</Body>
<Width>1.656in</Width>
<Page>
<PageHeight>21cm</PageHeight>
<PageWidth>29.7cm</PageWidth>
<LeftMargin>0.75cm</LeftMargin>
<RightMargin>0.75cm</RightMargin>
<TopMargin>0.75cm</TopMargin>
<BottomMargin>0.75cm</BottomMargin>
<ColumnSpacing>0.13cm</ColumnSpacing>
<Style />
</Page>
</ReportSection>
</ReportSections>
<ReportParametersLayout>
<GridLayoutDefinition>
<NumberOfColumns>2</NumberOfColumns>
<NumberOfRows>4</NumberOfRows>
</GridLayoutDefinition>
</ReportParametersLayout>
<EmbeddedImages>
<EmbeddedImage Name="legend_template">
<MIMEType>image/png</MIMEType>
<ImageData>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</ImageData>
</EmbeddedImage>
</EmbeddedImages>
<rd:ReportUnitType>Cm</rd:ReportUnitType>
<rd:ReportID>6203a576-d7cc-4b57-9a9b-12cf325d6203</rd:ReportID>
</Report>