Non-linear curve fitting problem

[JenniferMurphy]

I am trying to determine the equation that best approximates the values in this table for converting raw scores in bridge to IMPs.

Up To	IMPs		Up To	IMPs		Up To	IMPs
10	0		420	9		1740	17
40	1		490	10		1990	18
80	2		590	11		2240	19
120	3		740	12		2490	20
160	4		890	13		2990	21
210	5		1090	14		3490	22
260	6		1290	15		3990	23
310	7		1490	16		4000+	24
360	8

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I believe it is a logarithmic relationship, but I have some zero data. Should I set those values to something like 0.1 or 0.00001?

I have the data in two long columns in a spreadsheet (not broken into 6 as above). I select these columns and click Insert | Scatter. I get a nice graph of the data.

Next I right-click on one of the data points and select Add Trendline. With the zero values included, both the Exponential and Power options are greyed out, so I have Linear, Logarithmic, Polynomial, and Moving Average available. If I choose Linear, Polynomial, or Moving Average, I see a line on the graph and I am still in the Format Trendline dialog. If I click on Logarithmic, I am taken out of the dialog and there is a legend for a Log line, but no line is on the graph.

With the zero values included, the best fit is a 6th order polynomial. None of the others are even close.

With the zero values excluded, the logarithmic option gives a pretty good fit.

If I replace the zero values with 0.0001, then I can get all of the options, but they are all way off including the logarithmic.

Is there a way to get Excel to choose the "best fit" and work around the zero values?

Thanks