<<ark is a string variable containing the sheet name. This is part of a sub,
accepting different sheet names.
Ah ha. I didn't think about the fact that you might be using a variable.
Normally, you are supposed to use object variables to point to the object.
If the worksheet being referenced by the code is not the active worksheet,
then these types of problems result.

So, at the top of your code, you should add variables to reference the
worksheet and chart, then set them to the worksheet and chart, like so:

Dim ws as Worksheet
Dim crt as ChartObject
'Other variable declarations.

Set ws = Worksheets(ark)
Set crt = ws.ChartObjects("Chart 1")

With crt.Chart.PlotArea
'Code to set and check PlotArea properties.
End With

I had these problems a lot also in the early days of learning Excel VBA
macro programming. (Time to buy a couple of books or visit the
http://www.mvps.org/links.html#Excel page and see what they might have.)
--
Hope this helps (HTH),
Regards,
Bill