console: add “%” to available units

This is useful to detect interfaces that are close to saturation
quickly. It would usually require to group by exporter name and
interface name and it may not make sense for some graph types (like
stacked 100%). It is useful with Lines and Grid.

console/data/docs/03-usage.md

@@ -111,8 +111,11 @@ allows a user to explore data using graphs.
 The collapsible panel on the left has several options to change the
 aspect of the graph.
 
-- The unit to use on the Y-axis: layer-3 bits per second, layer-2 bits
-  per second (should match interface counters), or packets par second.
+- The unit to use on the Y-axis: layer-3 bits per second, layer-2 bits per
+  second (should match interface counters), packets par second, percentage of
+  use of the input interface or output interface. For percentage use, you should
+  group by exporter name and interface name or description for it to make sense.
+  Otherwise, you would get an average over the matched interfaces.
 
 - Four graph types are provided: “stacked”, “lines”, and “grid” to
   display time series and “sankey” to show flow distributions between