Below is the graph of a trigonometric function. It has a maximum point at $\left(\frac{1}{4} \pi,-2.2\right)$ and a minimum point at $\left(\frac{3}{4} \pi,-8.5\right)$. What is the midline equation of the function?

Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: Below is the graph of a trigonometric function. It has a maximum point at $\left(\frac{1}{4} \pi,-2.2\right)$ and a minimum point at $\left(\frac{3}{4} \pi,-8.5\right)$. What is the midline equation of the function?

Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: Below is the graph of a trigonometric function. It has a maximum point at $\left(\frac{1}{4} \pi,-2.2\right)$ and a minimum point at $\left(\frac{3}{4} \pi,-8.5\right)$. What is the midline equation of the function?
