I'm having problems with a User Defined function.

I have a chart (line graph) based upon a column of data calculated by a UDF.
I also have another cell that is the slope of a range of the column using
the SLOPE function.

Now, if for any reason I can't calculate a value in my column (e.g. some of
the parameters are missing) I'm not sure what to do:

1. If I return #N/A (Using CVErr(xlErrNA)) then my graph is OK, but the
SLOPE function also returns #N/A (Though the chart has a trend-line which
does have a slope calculated and displayed)

2. If I return "" then the line drops to zero

3. If I return Null then this is being taken as 0 and once again the line
drops to zero.

4. If I delete the formula from the cell then it appears as blank, and there
is a dis-joint in the line

So, how do I get the chart to appear OK (with no disjoint) without mucking
up SLOPE() and what is the difference between a cell with no value and a UDF
that returns Null (or a zero-length string)

Help. I'm very confused.

Rob