You're really close. I think all you need to do is remove OperationName from your chart command
| chart values(Duration) over txn_id
^ However, looking at your serach, I'm guessing that you actually want to use list(Duration) instead of values(Duration) since it looks like you want to know the value for each and every txn_id regardless of uniqueness. Using values will list the value if it hasn't seen it before (it is a unique list). So for example, if two of your txn_id's had the same value, like 5, you'd only see it listed once from the values() function whereas list() doesn't care and just lists out the values, even if things are repeated.