Sub myCode()


  Dim myChart As ChartObject, iRow As Integer
  Dim rng As Range
  Set myChart = ActiveSheet.ChartObjects(1)


  With myChart.Chart
      For iRow = 1 To 15
          Set rng = Sheets("C").Range(Sheets("C").Cells(26, iRow + 2), Sheets("C").Cells(51, iRow + 2))
          .SeriesCollection(iRow).Values = Range(rng)
          .SeriesCollection(iRow).XValues = "'C'!$A$26:$A$51"
       Next iRow
  End With


End Sub

.SeriesCollection(iRow).Values = Range(rng)

.SeriesCollection(iRow).Values = "=""" & Sheets("C").Range(rng) & """"
.SeriesCollection(iRow).Values = Sheets("C").Range(rng)
.SeriesCollection(iRow).Values = Range(rng).Value