[R] suggest appropriate model?

[steven wilson]

Dear R users;

I've been trying to fit the following set of data using something like
the sum of two functions (biphasic function) but without success. I'd
like to know if anybody can suggest a better or appropriate
expectation function to use. Here is the data:

x=c(298.15,308.15,318.15,328.15,338.15,348.15,358.15,368.15,378.15,388.15,
398.15,408.15,418.15,428.15,438.15,448.15,458.15,468.15,478.15,488.15,
498.15,508.15,518.15,528.15,538.15,548.15,558.15,568.15,578.15,588.15,
598.15,608.15,618.15,628.15,638.15,648.15,658.15,668.15,678.15,688.15,
698.15,708.15,718.15,728.15,738.15,748.15,758.15,768.15,778.15,788.15,
798.15,808.15,818.15,828.15,838.15,848.15,858.15,868.15,878.15,888.15,
898.15,908.15,918.15,928.15,938.15,948.15,958.15,968.15,978.15,988.15,
998.15,1008.15,1018.15,1028.15,1038.15,1048.15,1058.15,1068.15)
		
y=c(0,-0.12,0.05,0.44,1.01,1.71,2.44,3.14,3.73,4.2,
4.53,4.78,4.95,5.08,5.22,5.37,5.53,5.73,6.02,6.42,
6.99,7.77,8.84,10.37,12.7,16.13,20.38,24.61,28.47,32.17,
36.16,41.01,46.59,51.22,53.98,55.92,57.53,58.85,59.96,60.96,
61.87,62.7,63.46,64.17,64.82,65.4,65.93,66.41,66.85,67.23,
67.58,67.89,68.19,68.47,68.73,68.98,69.2,69.43,69.64,69.84,
70.03,70.22,70.41,70.6,70.79,70.99,71.2,71.46,71.73,71.96,
72.17,72.38,72.58,72.78,72.98,73.18,73.39,73.59)	

plot(x,y)

Thanxs
SW