All shapes (including Charts) in Excel have AlternativeText property. This property can be used to store some information as well. Following is the code to read and write to AlternativeText property of a shape.

Sub SetAlternativeText()
 
    Sheet1.Shapes("Rectangle 1").AlternativeText = "ABC"
 
End Sub
 
 
 
Sub GetAlternativeText()
 
    MsgBox Sheet1.Shapes("Rectangle 1").AlternativeText
 
End Sub