The bottom option in the date range pick box is "custom date range". I pick that and it accepts a custom date range. Is that what you are seeking? Or are you looking for the custom date range for the average spend data? In that case you can only pick from the finite set of range options. And, If that is what you are seeking then you'll need to do the manual workaround. However, doesn't choosing a custom set of dates to create the average essentially defeat the concept of average?
Have you considered creating a budget instead of comparing against averages. The comparison to averages becomes a moving target unless you reference a fixed time period in the past, in which case creating a budget and comparing to budget might be easier and more useful, especially since the budgeting features are relatively well built out.