display a warning if there were no updates within a regular interval

[grandrew]

As we are dealing with real-time data we need to make sure that we have the feeder attached.

Some visual indication of data update status is required

• [ ] ability to show last update time
• [ ] show warning sign if no updates registered in timeout based on last data update values
  • [ ] show a fading tip that the data was not updated
  • [ ] show last update time permanently in case of warning

also see #16

[dpq]

Perhaps simply hueing down plots and adding a hourglass icon/time since last update will suffice?

[grandrew]

or even something like this: