Formula of frequency modulated sine wave from data points
If your function is $y(x)=\sin(2\pi fx+C)$, and you have data points $\{x_i,y_i\}$, then finding $C$ is easy, if you suppose $x=0$ then $C=\arcsin(y(0))$.

Now, according to your plot your frequency varies with $x$. The first thing you need to do is to know how does your frequency vary with $x$. Supposedly this variation is linear, then you write: $f(x)=ax+b$ and your function model becomes $y(x)=\sin(2\pi f(x)x+C)$.

The problem now is finding (an estimate for) $f(x)$. You have two unknowns to determine $a$ and $b$, you need two data points ($\{x_1,y_1\}, \{x_2,y_2\}$) and you solve a system of two (nonlinear) equations with two unknowns.

$$\begin{cases}
y_1=\sin(2\pi (ax_1+b) x_1+C)\\
y_2=\sin(2\pi (ax_2+b) x_2+C)
\end{cases}$$