Hi,
You have to use the X-Axis mode=Series instead of X-Axis mode=Time if you always wants to have 7 days nevertheless selected time range. You find this setting in panel edit mode and the Axes tab. But to be able to do this you need to use a query that will return a series per day.
Have a look at the following example panel: http://play.grafana.org/d/000000016/graph-styles?panelId=19&fullscreen&edit&orgId=1&tab=metrics
Marcus