I've been wrestling with this, and have decided to set charts so that the user can then control their view. I include a Timeframe set of buttons so users can view the last 7 days, 14 days, MTD, YTD, last 12 months and All time, with the dimension changing from day to month to year as appropriate (driven by variables). If filtering on a customer shows too widespread/little data on the week view, the user can manually switch to MTD, YTD etc.
Using a combo chart, current values are shown as a bar, and last week, last month, last year values as lines (droping last week and month when not relevant). A Compare To set of buttons can then colour-code the current bars to show whether they are better or worse than the last week/month/year.
It's tricky to set up, but can be applied to most areas off functionality.