Declaring ActiveChart Object

The active chart is a chart object. The parent of a chart object is the chart object collection. The parent of that is the sheet.

If you want to create a new chart:

Sub AddChartObject()
Dim myChtObj As ChartObject
'
    Set myChtObj = ActiveSheet.ChartObjects.Add _
        (Left:=100, Width:=375, Top:=75, Height:=225)
    myChtObj.Chart.SetSourceData Source:=Sheets("Sheet1").Range("A3:G14")
    myChtObj.Chart.ChartType = xlXYScatterLines
End Sub

If you want to manipulate an existing chart

Sub SetChartAxisScale()
Dim ws As Worksheet
Dim myChtObj As ChartObject
    Set ws = ThisWorkbook.Worksheets("ITF_Chart")
    Set myChtObj = ws.ChartObjects("Chart 3")
    With myChtObj.Chart.Axes(xlValue)
        .MaximumScale = 44
    End With
End Sub