Need hint about finding the relevant code in "chart2"

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!