Variable Range For Chart Using VBA

I recorded a simple macro to create a chart using VBA. The code for the macro is:

Code:
Sub Add_Chart1()
'
' Add_Chart1 Macro
'
ActiveSheet.Shapes.AddChart.Select
ActiveChart.ChartType = xlLine

[code]....

The problem is that I need the code to select a variable range from C4 to the last used row in column C for the x values and from G4 to the last used value in column G.

I tried to modify the code to do that by doing the following:

Code:
Sub Add_Chart1()
'
' Add_Chart1 Macro
'
ActiveSheet.Shapes.AddChart.Select
ActiveChart.ChartType = xlLine

[code]....

When I run the macro, I get an error 91, "Object variable or With block variable not set"; Also the chart is created but not correctly.

I created a sub called "range" (see below) and it correctly selects selects the C or G ranges, however, the argument does work in the chart macro.

Code:
Sub Range()
Sheets("Sheet1").Range("C4:C" & Sheets("Sheet1").Range("C" & Rows.Count).End(xlUp).Row).Select
Sheets("Sheet1").Range("G4:G" & Sheets("Sheet1").Range("G" & Rows.Count).End(xlUp).Row).Select
End Sub