finding a fuction of set of refined points

I have a set of points, time and RPM of a rotating crankshaft. And the RPM is refined to .0000001 time scale resulting in a set of 4000+ points. The graph is plotted below. Is there any method to find the function of this graph?(which can be further applied in Multibody simulation to replicate the test bench). And the RPM gradually decreases and come to halt after the ignition is turned off.

[Stephen23]

"Is there any method to find the function of this graph?"

There are infinite functions that exactly fit any finite set of data points. Which of these infinite functions are you wanting?

[Srimanish Reddy Enakonda]

I would like to find a function in which the small periodic fluctuations (can be considered as a straight horizontal line) till the rpm drops are same until the ignition is turned off. A simple expression which satisfies the graph and can be used without complications in a multi-body simulation to obtain similar results.

[Stephen23]

You basically have a choice between:

1. you know something about the underlying physical process, and can use this to fit a model to the data. Any internet search engine can help you: "MATLAB curve fitting".
2. you know nothing about the process (or it is too complex), then you can fit a spline. Any internet search engine can help you: "MATLAB splines".