So you are using some software tool to draw the diagram?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
I use sequence diagrams a lot and I wanted to pick up on your concern about which way the arrows run. I find it more pleasing when calls all go left to right and returns all go right to left, but it can't always be done. Callback methods come to mind as one exception. Other cases might expose circular dependencies and may indeed be bad ... or not. Anyhow, I wouldn't worry that there is any rule against calls going right to left.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi