Missing values in bar graph with wrong values showing wrong graph shape

In stacked bar graph (and other types) when there is missing values it keeps the last known value, I would like to get a 0 value instead otherwise the bar graph global view is incorrect (showhing holes with a wrong value). I tried null values=null or =0 in “stackeing & Null value” but none work.
Something to do to get good graph shape ?
grafana 5.4.3

See black holes under bars.