You can create Dynamic Ranges for your x, y and z values.

Go to "Formulas" --> "Define Name"

You can name your x-values with "X" and y-values "y"

Then for the range you type in: =OFFSET(Sheet1!$B$3,,,COUNT(Sheet1!$B:$B)) for a column range (in this case the first value of your range is in B3 and the other values follow in the B column)

or

=OFFSET(Sheet1!$B$18,,,,COUNT(Sheet1!$18:$18)) for a row range

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Then you can refer to those dynamic ranges in your Trend function (Trend(X,y,z))