Custom graphs: Changing default "Last 48 hours" timeframe

I need to look at my custom graphs with a higher resolution (shorter
timeframe) than the default "Last 48 hours".

I tried the changes below in hobbitgraph.cfg (appended ":start=end-4h"
to the DEF entries), but this didn't make any difference.  I was
attempting to get a "Last 4 hours" display.

...
[recent]
        TITLE Incoming Cases
        YAXIS Count
        DEF:five=recent.rrd:five:AVERAGE:start=end-4h
        DEF:ten=recent.rrd:ten:AVERAGE:start=end-4h
        DEF:fifteen=recent.rrd:fifteen:AVERAGE:start=end-4h
        DEF:twenty=recent.rrd:twenty:AVERAGE:start=end-4h
...

I also tried "end=start+4h", not expecting that to give me what I
wanted, but as a test to see if I could get a four hour window
displayed.  This directive appeared to be ignored as well.  The display
always shows "Last 48 hours".