add function `set_x method` with (start: f64, end: f64) to make better experience for set_x(&self, &PlotBounds)

**Is your feature request related to a problem? Please describe.**

When developing with egui 0.34, it makes mad when i only need to change x while i need to pass the whole struct

**Describe the solution you'd like**

add a method of set_x_bound(&self, start: f64, end: f64) to do things in set_x(&self, &Self)

**Describe alternatives you've considered**

**Additional context**