I'm trying to fit a 3rd degree trend line to a data series with independent
values starting from -8 to 10. The 3rd degree equation that I get keeps
assuming that the first independent data point is zero, and so, the
y-intercept of the equation is not the value for -8. These are the data
points,

m Freq (cm-1)
-8 2107
-7 2112
-6 2116
-5 2120
-4 2124
-3 2128
-2 2132
-1 2136
0 2139
1 2147
2 2151
3 2155
4 2158
5 2162
6 2166
7 2169
8 2173
9 2176
10 2180
And so, the y-intercept at x=0 should be close to 2139 instead of the 2104
value that I get. How to make excel recognize that the first data point is
not zero?
Please help,
Juan