Add examples for multi-series

Examples/Charts/MultiSeries.ps1

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+rm temp.xlsx -ErrorAction Ignore
+
+$data = invoke-sum (ps) company handles,pm,VirtualMemorySize
+
+$c = New-ExcelChart -Title Stats `
+    -ChartType LineMarkersStacked `
+    -Header "Stuff" `
+    -XRange "Processes[Company]" `
+    -YRange "Processes[PM]","Processes[VirtualMemorySize]"
+ 
+$data | 
+    Export-Excel temp.xlsx -AutoSize -TableName Processes -Show -ExcelChartDefinition $c

Examples/Charts/MultiSeries1.ps1

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+rm temp.xlsx -ErrorAction Ignore
+
+$data = @"
+A,B,C,Date
+2,1,1,2016-03-29
+5,10,1,2016-03-29
+"@ | ConvertFrom-Csv 
+
+$c = New-ExcelChart -Title Impressions `
+    -ChartType Line -Header "Something" `
+    -XRange "Impressions[Date]" `
+    -YRange @("Impressions[B]","Impressions[A]")
+
+$data | 
+    Export-Excel temp.xlsx -AutoSize -TableName Impressions -Show -ExcelChartDefinition $c