Also discussed here https://www.reddit.com/r/kustom/comments/6gxv7p/data_usage_widget/
Basically data usage is only been reported for the current month, even though I set it to grab from the 20th of last month to the 20th of this month it is missing 10 days of data usage
The formula provided on reddit is close to the correct one (i replaced r0d with 20d):
$mu(round, ts(mt, m, if(df(d)>=20, 20d, 20dr1M), r0d), 1)$,
$mu(round, ts(mt, m, if(df(d)>=20, 20d, 20dr1M), r0d), 1)$
This should work, please note that KWGT starts storing traffic info since first use so if you install it from scratch