I tried to use your IRR cell formula but only got #N/A out of it.

Maybe try changing -1 to 1 in the last match arguments?
The spline formula agrees with the Excel curve in many
practical situations. It won't match if the data spacing is
too irregular however - given four consecutive points
A,B,C,D then to apply in the interval BC, two conditions
must hold: |BC|<3|AC| and |BC|<3|BD| (in chart units.)


What "issues" with my function does your solution resolve?

The scaling adjustment can differ slightly in small intervals
and near endpoints - see example. The simplification comes
as you don't need the actual pixel lengths for the chart
scaling factor only ratios i.e. plot area width/length. My
UDF reads chart values so you'll need to update by pressing F9.

Example: In your smooth_curve_bezier_example file entering:

X Y
9 10
9.5 12
10 12
12 7

http://wikisend.com/download/544426/...ample_file.xls

BTW I think the ChartTool Utility is a great tool.