I would like to know ScreenCoordinates through RectTransform of DivGraph seen in the picture above.
Here, the above ScreenCoordinates is, it refers to the same as the value that can be obtained through the Input.Mouse.Position like.
But no idea how.
If I understand your question correctly, then camera.WorldToScreenPoint is what you are looking for. You need to provide it a world position as parameter, so either rectTransform.position or any of the Vector3 returned by rectTransform.GetWorldCorners will do.
You need to have a reference to a camera to get screen position. Because if more then one camera renders it, you could potentially have multiple different screen coordinates for the same object.
If you are using a canvas set to Screen space overlay, there is no need to do this, as in this case, the world position is equal to the screen position (Unity scales the canvas so that 1 pixel is equal to 1 Unit in Unity, therefore there is no need to convert between screen and world positions). so rectTransform.position is in that case also your screen position in pixels.