You can try adding two dimensions and both the expressions in a bar chart. But without knowing how they look or what kinda resultant graph you are looking for its tough to answer. As stated by @Manish please attach a sample doc.
The way I would do it, create a separate resident table from 'Cases' like
Create_Date as Date,
'Open' as Condition
Close_Date as Date,
'Closed' as Condition
Drop field Condition from Cases;
Make sure PKY is a unique field in Cases table. You can use rowno(). Then use this 'Date' field in the dimension and the two expressions you already have in the same chart and you can include Condition in the Set to determine which is opened/Closed.