Lookups & removing errors while using linest

[catgeo]

Hi,

I'm using linest to obtain the slope of a series of data.
My data is three colums of X, Y and 'category'.

e.g.
X Y 'category 1'
X Y 'category 1'
X Y 'category 2'
X Y 'category 3'
X Y 'category 3'
X Y 'category 2' ect. ect.

What I'm trying to do is look up the specific categories X and Y variables and find the slope of that 'category'.
So far I have got to the point where my linest function if calculating the slope of a X & Y array that contains the categories data but the rest of the array is zeros or an error. I need a way to filter out the zeros/errors.

My code so far is (or a variation of):

=LINEST( "X-array" *(( "category array" = "category n" )) , "Y-array" * (( "category array" = "category n")))

I've also tried using the aggregate (small) function, but after the aggregate (small) function calculates it returns the smallest number in the array and not the entire array, which I need for linest to calculate with.

Can anyone please help?
Thank you for you help

[XOR LX]

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