xyChart: minor visual tweaks

[s-h-a-d-o-w]

All other things I've found related to xyChart are about new features but personally, I would appreciate just some pretty straightforward (I'm guessing 😅) visual tweaks.

1. Have the gap not just between bars but also around. It looks kind of weird to me how the bars end flush with the chart:

2. Make it possible to specify the number of ticks. Reason being: When using slightly larger fonts, especially on horizontal charts, it is currently very likely for large numbers to overlap.
3. The first tick along the value axis should either be in line with the other axis or not be rendered. Because it looks pretty strange when in a chart depicting only positive values, 0 isn't where the axis starts.