This depends on your RRD configuration, and your frontend. Lets assume you are using routers2 as your frontend.
If the data is becoming unreadable, you might try the 'rescale' button, which tries to change the y-axis to cope with this sort of problem.
If you have the rrd-archive script enabled (in the extensions directory), then you can select previous archived rrd databases so that you can see the 5-min-interval daily graphs from previous days.
If you have 'extended time' enabled (you may need to modify your .rrd files for this) then you will get a 'yesterday' option as well as 'daily' to give you the previous day.
The problem with generating a graph for an arbitrary time window is that usually, this data is simply no longer held in the .rrd. The format of the rrd will summarise and expire data constantly, so after a couple of days we can no longer access data at a 5-min granularity (which is what the 'daily' graph uses). You end up with weird blocky graphs as it tries its best with what it has.
The reason why almost all frontends for MRTG use the daily/weekly/monthly/yearly graph set is that these correspond to the default RRA (summary archives) defined in an MRTG-generated .rrd file. Although you can define different RRAs, this would have to be done manually.
This is detailed in the FAQ for routers2, since people ask for this frequently.
http://www.steveshipway.org/forum/viewtopic.php?t=43
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