Allow controlling the gauge/history visibility, and the history
interpolation method, of individual values.
Improve redraw elision when some values are hidden.
GimpMeter visualizes a set of values that change over time. It
consists of a gauge, showing the most-recent sampled values, a
history graph, showing a plot of the values over time, and an LED,
which can be used as a boolean indicator for some condition.
GimpMeter is used in the dashboard dockable, added in the next
commit.
Michael Natterer