The filter is not on a chart object but rather the underlying data model as a whole. You can filter a field from any of the chart objects displayed. If you had the chart title you filtered from and included it in the name the selection displayed it would be long and misleading as in effect filtering on country=United kingdom is filtering your entire dashboard not just the chart to chose to click on United Kingdom to filter.
Hope that makes sense.
Andy - it does make sense but then again I feel derived fields are missing a rename functionality.
In my example, if you were to filter on a derived calendar field (e.g. DueDate.Calendar.Year), that is the field that will show in the selection bar - and it can (will certainly) be truncated.
While the idea of derived fields is good, I prefer to define year, month and other date-parts in the LOAD script as separate calculated fields just to keep control on the names.
@Qlik - care to comment if the DERIVE function would allow explicit derived field names in a future release ?
I am finding this issue difficult as well as the field or expression also comes up when you use getcurrentselections() too. So instead of for example,
Reporting Year: 2015
if([Reporting Year}>2002, [Reporting Year], Null()): 2015
which my user doesn't need to see, I can't build this into the script as the other data is needed elsewhere in the app.