I want to perform a 4th order curve fit on some values in an array.
I have a list of X values and a list of Y values, the X values are not in
any particular order and there are in some cases multiple instances of a
given X value. I want to make a curve fit on the Y data only when the
corresponding X values are between some set limits, set in seperate cells.

I have tried (where my limits are $A$1 and $B$1):

{LINEST(IF(X10:X100$A$1,IF(X10:X100<$B$1,Y10:Y100 )),IF(X10:X100$A$1,IF(X10:X100<$B$1,X10:X100^{1,2 ,3,4})))}

This gives #VALUE if the limits do not encompass the whole data X range. I
guess because this results in non contiguous data in the array...

Any smart ways around this?