The function appears to be a quadratic function displaying an opening upward parabolic shape.

[CODE]:
```python
def approximate_function(x):
    a = 0.5
    b = 0
    c = 0
    return a * x * x + b * x + c
```
This is a simple approximation based on the observed behavior. You may need to adjust the parameters (a, b, and c) to achieve a better fit with the actual function.