Don't disable L1 labels when last datum not-in-view

[goodboy]

I changed my mind on this since the downsampling magic landed in #302 😂
The latency for redraws of flows is now so low (and the coord caching seems to be working better on newer versions of Qt) that I think we can add this label update stuff back in.

It's actually pretty handy to see the current L1 spread on the y-axis when looking at history there's just some caveats we should ack and enforce:

• obviously don't auto-scale the yrange to include the L1 / price when looking at history that is not in that range, i.e. the history range takes precedence

  • in the case where L1 is off-view, it'd be nice to have an indicator arrow pointing to where it is (top or bottom) much like we have for pp lines.

• only pause the feed if panning on large data sets?

  • not sure this "optimization* is revelvant any more

  Probably pertains to HF feeds and UI performance via throttling #192, Fixing the convoluted ._set_yrange() callbacks mess #343