oops, was reading too fast. sorry.
The scale is dynamically set based on the time period selected, the frequency and the size of the chart. If you change any of those, the increments on the X axis will change as well.
As for afterhours data - right click on the chart, select VIEW menu and select to NEVER show Pre/post market data. However, that would make the first increment 9:30, not 9:00 since the markets open at 9:30