We had an incident recently where a specific statement executed a very unusually high number of times over a couple of days, and experienced a lot of waits. This has skewed the scale of the chart so badly as to make it useless until those days "age ouf" of the 30 day window.
Being able to add those days to a list of exclusions (with the ability to toggle the exclusions as a set, much like in the displays for shorter time periods ("There are days that may have been excluded from this chart. View excluded Days") would be very helpful.
I would suggest implementing this by adding a toggle below the day view of this chart ("Show this day in All Days Trend chart" much like the one in the SQL Properties dialog: