Show usage against the plan limit for the current billing period #8185

Goal. Someone opening Usage can tell how much of their plan they have used, without doing any arithmetic.

Problem. The page reports totals for whichever window you picked. It never says how close you are to the plan limit.

Scope

• default to the current billing period
• "% of plan consumed" as the headline, and as a tile
• the usage bar, with the notification markers on it
• cumulative usage against the plan limit, with the limit drawn as a ceiling. Billing periods only, see below
• the slot under the meter for the line that explains the numbers. Its wording is a projection (Show projected end-of-period usage #8188) or an overage consequence (Build the over-limit and restricted states for the usage page #8187), so this issue builds the slot rather than the sentences

Covers the healthy and approaching states. Over limit and restricted are #8187.

Not here

• the projected end-of-period figure, in the tile, the chart line and the slot. All Show projected end-of-period usage #8188
• the period's dates, its reset date and days remaining. All Display the billing period dates on the dashboard #8258, because none of them can be read from the API today

Free plans

Measured over a trailing 30 days, so there is no period start and nothing to project to. They get the meter and daily volume rather than cumulative against a ceiling, because a rolling total can fall as old days drop out of the window.

No plan limit

Where there is no limit to divide by, show the total and no percentage.

Done when someone on any plan can see how much of their allowance they have used.