Raw browser issues (and ideas)

I think that a couple of features might be missing from our raw browser (not specific to any backend):

1. There is no indication of the absolute values of a channel, because the scale bar only shows a range. It would be very helpful if e.g. the zero value (or min/max) was shown as well.
2. The browser always shows "0.0 AU" in the scale bar for "stim" and "misc" channels (and possibly other types).
3. There is only one scale bar even if there are two different channels types (e.g. "misc" and "stim").
4. Could we automatically show the best unit prefix (e.g. instead of 20,000µV -> 20mV)? This would make the scale bar easier to read for more exotic scalings.

Here's an example which demonstrates these issues:

import mne
from numpy.random import default_rng

fs, nchans = 250, 3
rng = default_rng(42)
data = rng.standard_normal(size=(nchans, 50 * fs)) * 5e-6
data[0] += 10
info = mne.create_info(nchans, fs, ["eeg", "misc", "stim"])
raw = mne.io.RawArray(data, info)
raw.plot()

1. The first channel has an offset of 10V, but this is not apparent from the plot.
2. The second channel has a scale bar with "0.0 AU".
3. The third channel does not have a separate scale bar, although it has a different type than the second channel.

Start by running the provided Python example with mne.io.RawArray and raw.plot() to reproduce the scale-bar behavior. Trace the raw browser entry point and assess the four requested behaviors: absolute values, nonzero units, separate bars for channel types, and automatic unit prefixes. Done means the requested scale information is displayed correctly for the example without backend-specific assumptions.