Here is what a typical output looks like.
And this happens after I dispatch thousands of construction robots deconstructing an area with lots of supplies (moral of the story: build much more accumulators).
How does it work? The combinator on the lower right with green wires is a clock, as it is described in the tutorial on the wiki. The output of the battery is connected to the column of lamps, which light up when the battery level reaches certain thresholds (10%, 20%, ..., 100%). The other combinators make up the shift register that shift the output left each time the clock resets itself.
This has a fairly good resolution, but slows down the game significantly while it is visible. So for longer periods, I made another one that displays daily minimum levels.
The shift register is the same, but there is a somewhat more complex logic at the input. The clock is set to one day. The other combinators read the input, and holds the lowest value encountered. It is done by holding the output when it is lower than the input, otherwise outputting the input. When the clock resets, a pulse of 100 is added to the output, resetting the minimum.
Here are the blueprints.
normal
daily minimum