add a way to indicate when a point is off the scale

New feature request: it would be great if there were a way to hint when a point is off the scale.

• With bars, it's already possible - the top of the bar has no border. However, this is quite subtle, and perhaps it could be made more obvious that it's truncated, e.g. by making the top few mm of the left and right edges end with dashes and an arrow-tip.
• With bubbles/scatter points (that are not connected by lines), there is no way to see that a bubble is too high (rather than being omitted because the point is null).
• This is particularly problematic when the user has manually set the y-axis range, and has cut off a point. (in auto-range mode, points are never missed out).

This might be indicated by showing a "^" at the top of the paper pointing to where the point would be. (such a symbol should also get the hover-text of the associated point).

These indicators should be dynamic - as the axis is zoomed and scrolled. Maybe we need a pref (default on) to turn this off?

Thanks for your consideration - I hope the idea is useful.

No files or tests are named in the issue. Start by tracing the bar and scatter/bubble rendering paths alongside axis zoom and scroll updates; done means clipped points have clear, dynamic indicators with associated hover text, while null points remain distinguishable.