The original note, as written at the time
This app would genuinely track the data usage on your device to determine when the data provider is giving you less data or misusing your data with their own arbitrary activities. This would also let you know which apps are carrying out arbitrary activities with your device — for example during updates you would be able to know which apps are instead getting large chunks of information from your device.
Ongoing development
An app that genuinely tracks the data used on a device, in order to determine when the provider is giving you less than you paid for, or using your allowance for its own purposes.
The word is genuinely, because the phone already tells you a number and the number is the operator’s number. There is an asymmetry at the heart of every data bundle: the seller measures the goods and the buyer has no independent instrument.
An independent meter is not a complicated thing. It counts bytes at the interface, and it compares them to the bill. Over a month, the difference between what the device sent and what the operator charged is either explained by something legitimate or it is not.
The second half of the note is the more interesting one: knowing which apps are taking large chunks of your device rather than simply updating. Every update is a request for permission to move an unknown quantity of data, and the user has no way to distinguish a security patch from a distribution of advertising assets.
Which turns the meter into a form of literacy. After a month of seeing it, a person knows what a video costs, what an update costs, what a background sync costs — and knowing changes behaviour more than any warning does.
The design constraint is that it must be readable by a person who is not an engineer. Not megabytes per second, but: this call used a tenth of your bundle; this app used a third of it while you were asleep.
A meter the buyer owns is the beginning of an honest market.