Make getting started first figure have integer x axis ticks

[seabbs]

          ![image](https://github.com/user-attachments/assets/eab1b4a6-2bb6-4ff7-a3ec-a1d166857c22)

Given that this is a discretisation should the x-axis here be 0, 1, 2, ...?

Originally posted by @athowes in #43 (comment)

[seabbs]

I just looked ascross the vignettes and this crops up a few times. I think that for consistencies sake and because in the vast majority of these plots something continuous is also being plot the answer is no and the axis is okay being continuous (and so free to pick its own ticks automatically). I'm happy to hear push back on this but if we do make a change I think it should be everywhere. Closing for now but as above reopen if desired.