Include 0 in x-axis density plots if within some proportion of the range of values.

[rambaut]

I.e., if the range is 25 - 100 then this range is only 25% smaller than 0 - 100 so we should include 0 on the axis. This should be true for ordinal traces too.

[rambaut]

This has not been implemented:

The above density is very close to 0 but still uses a truncated x-axis.

Not urgent.