Need hint about finding the relevant code in "chart2"

[walt]

My linux distro just dropped support for 3.4.x and now defaults to
3.5.4.x.  Somewhere between those two versions, the behavior of chart2
changed in a way I don't like (I don't want to call it a regression,
exactly ;)

I suspect the code I need may be in view/axes/* but I'm not sure
because there's a ton of stuff in there I don't understand yet.

I need to find the code that parses my choice of "Data Ranges" for
the horizontal axis of a stock chart -- and now refuses to let me
do what I've always done -- leave blank space at the right end of
the chart, to allow for fresh samples to be added each day as I get
them.

Any hints would be very welcome, thanks!