The title defaults to field if not overridden but if you want to give another "display" name on the sheet, should QS not been consistent and use that name/title in the selection bar too ?
I understand the field can be renamed in the load script but then what's the point for the title. And sometime it is not possible: if you declare a "Calendar" field definition and derive a date field e.g. DAT to Year, Month,... you end up with compound fieldnames that you cannot change.
As a matter of fact, even if I renamed a DAT.Calendar.Year to "Year" in the master items, the selection still shows the original field name.
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.