Can you identify the "interesting" earliest/latest time and pass them as variable using the | map command?
Or if you only need a custom length sparkline with a special start/end condition (e.g. value larger than 100), you can edit the sparkline field using mv.... commands. E.g. Leaving only 10 tailing zeros at the end of the sparkline (fieldname=spcount and sorry about the regexp):
| nomv spcount | eval spcount =replace(spcount,"(##__SPARKLINE__##,[0-9,]+[1-9]{1},)\S+(0,0,0,0,0,0,0,0,0,0)$|(##__SPARKLINE__##,[0-9,]+)","\1\2\3") | makemv delim="," setsv=true spcount
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