Status Chart Bad Quality Data

I use a status chart to show a machine status (4 states: off, auto, manual, fault). The chart data are binded to a tag history and it is set in order to show last 8 hours realtime data.
When the system is running, so when data from the machine are available through OPC connection, everything works as expected. The problem rises when data are not available, e.g. when connection between ignition and the machine is not working: even if the Ignore Bad Quality setting is checked, the chart shows random values, between the 4 programmed, for the period with no data stored.

Thank you.