This first bit finds the last date cell.


Option Explicit
Function FindLastDateCell() As String

Dim c As Integer, lttr As String

Range("B2").Select

    Do Until IsDate(ActiveCell) = False
        ActiveCell.Offset(0, 1).Select
        c = c + 1
    Loop
    c = c + 1
        lttr = Split(Columns(c).Address(1, 0), ":", -1, 1)(0)
    FindLastDateCell = lttr
End Function
This bit calls the first one and then substitutes "cell" with "lttr" which is the last date cell.

Sub PORTMPDE()

    Application.ScreenUpdating = False

    Dim i As Long, indx As Long, a As Long
    Dim rng, rfrnce
    Dim lttr As String
     
    Worksheets("PORT MPDE").Activate
    
    lttr = FindLastDateCell
    
    Worksheets("Trending Charts").Activate
      
    rng = "A8:cell8;A9:cell9;A11:cell11;A12:cell12;A13:cell13;A14:cell14;A15:cell15;A17:cell17;A23:cell23;A28:cell28;A29:cell29;A30:cell30;A31:cell31;A32:cell32;A33:cell33;A34:cell34;A35:cell35;A36:cell36;A37:cell37;A39:cell39;A40:cell40;A41:cell41;A44:cell44;A45:cell45;A46:cell46;A47:cell47;A48:cell48;A49:cell49;A57:cell57;A58:cell58;A59:cell59"
    rng = Replace(rng, "cell", lttr, 1, -1, 1)
    rng = Split(rng, ";", -1, 1)
    
    indx = UBound(rng)
    
    rfrnce = "='PORT MPDE'!$B$2:$cell$2"
    rfrnce = Replace(rfrnce, "cell", lttr, 1, -1, 1)
     
    With ActiveSheet
        For i = 0 To indx
            With .ChartObjects("Chart " & i + 1).Chart
                .SetSourceData Source:=ThisWorkbook.Sheets("PORT MPDE").Range(rng(i))
                .SeriesCollection(1).XValues = rfrnce
            End With
        Next
    End With
End Sub