I think, reading between the lines, the issue may be that the OP wants to include something like snow in a WeeWX generated data file to be used by Highcharts, but using cm instead of mm as (seemingly) used for rain. I would expect that instead of passing something like '23 cm' in a single field Highcharts would want the value 23 in one field and 'cm' in another. The former can be obtained (for example) via
$day.snow.cm.raw or
$day.snow.cm.format(add_label=False) but there is no tag to get the unit label for 'cm' when your default unit for
group_rain is mm. The only solution I can think of short of digging into the WeeWX internals is some in-line python code that parses the result of
$day.snow.cm. Or hardcoding cm, but that is rather limiting.
Gary