auto bounds X/Y indepented

[gravl4]

X and Y was link in autobound mode.
problem example:
y 1.0 1.0 1.0
x 17777780000 17777880000 17777980000 (as timestamp in us) autobounds Y is 10000000

• Closes https://github.com/emilk/egui_plot/issues/THE_RELEVANT_ISSUE
• I have followed the instructions in the PR template

[oscargus]

I see that the current behavior may not make much sense, but the change is basically introducing dead code.

Look at the first change, it will only happen if bounds.width() <= 0. With your change, you use is_valid_x(), which is
self.is_finite_x() && self.width() > 0.0. Hence, is_valid_x() will never be true in this branch, so it will always take the other branch and set it to 1.

One may argue what the "best" width is to set here though. The current guess is that if width is negative or zero, set it to height (which gives an aspect ratio ~ 1), but there are clearly cases where it is a bad choice. On the other hand, always selecting 1, which is the current approach, may be just as bad in some situations.