How to get the equation of a graph after i have plotted the datas?

Hello!

I have a doubt and i need a quick answer please!!
I need to know if its possible to get in R the mathematical equation of a 
graph that you have plotted. I mean i know the y and x values, but i want 
the equation that relate them and that allow me to get the graph.
This option is possible in Excel, but i have  too many datas and i cant 
plotted them on it.
If its possible in R, how?


Thanks!!

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