From specific cell to last cell of that column

Is it possible to not specify an ending cell in functions such as Average, Sum, Sparkline or even charts?

Say I have a column of data from A10 to A20, i.e. does NOT start at row 1 so I can't use A:A. I'm continuously filling in more data so it might expand from A10 to A30. How do I use functions on this range of data with a changing ending row number? For example how do I create a sparkline from A10 to the last number in the column? Sparkline(A10:[last number]) so to speak.

http://i.imgur.com/azNnHGF.png

You could try something like this:

=SUM($A$2:INDEX($A$10:$A$100,COUNTA($A$10:$A$100)))

starts from A10 and considers down to row 100. can be adjusted...

Thanks, but I'm having trouble referencing this from another sheet.

For example, this will return parse error:

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but this does work:

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You never mentioned anything about another sheet....

=SUM(INDIRECT("'Historical data'!$A$10:A"&INDEX(Historical data!$A$10:$A$100+ROW(Historical data!$A$10)-1,COUNTA(Historical data!$A$10:$A$100))))

OR this

Formula:

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You can also use this formula for SUM, AVERAGE, MIN, MAX and COUNT functions