The line started last October last year at a low level, and then leapt to a point 14 times higher by this March. What the 3 mobile operator was showing a group of journalists and telecoms analysts was the flow of data over its network.

Now 3 has been a bit of a disaster, pouring billions into building Britain’s first 3G network, then twiddling its thumbs nervously as customers used it mainly for calls, rather than video or web-surfing.

So what’s behind the sudden explosion of data use? One word: dongles, those plug-and-play devices that give your laptop mobile broadband wherever you go.

3 says that while users may have been downloading a million music tracks a month, that involved minimal amounts of data compared with the industrial quantities consumed by home workers now plugging their laptops into the mobile internet.

And 3 is not alone – Vodafone has seen data use on its network surge since it started pushing dongles.

What’s driving all this is that the 3G networks are getting faster. 3 claims its network, which is now merging with that of T-Mobile in the UK, can deliver 3.6 Mbps right now, and will accelerate to 7.2 Mbps later this year and to 14.4 Mbps by the end of 2009.

With those kinds of speeds on offer no wonder there’s excited talk of mobile broadband competing with fixed line.

But hold on a minute – two things need to be sorted: price and speed.

We all know that the speed claims by the fixed line operators have been, well, dodgy. Kevin Russell, 3’s UK chief executive, describes the advertising practices of the broadband industry as “not much better than estate agents or second-hand car dealers.”

He also concedes that that his 3.6 Mbps network will only deliver between 1 and 2 Mbps to users. I’ve been testing one of the dongles, and have found coverage pretty patchy and slow – and of course indoors it can disappear completely.

Then price. A Google executive was at the same event pressing home the message that the mobile industry had to make its pricing much simpler if the mobile internet was really going to take off.

He put up a slide showing an advert from one mobile operator explaining a supposedly simple data tariff:

“When it comes to understanding the costs [we] have made things easy. Basically, you’re charged per page you look at, not per minute spent browsing. Each page costs between 1p-5p, depending on the number of images it contains.

“Browsing and downloading is charged at £x per megabyte (a megabyte being equal to roughly 250 pages).”

Point taken. Anyone reading that would have no idea what it was going to cost them to surf on the move.

The operators know that flat-rate, all-you-can-eat data tariffs are the future – but they are still worried about just how much we will want to eat and whether their networks can cope.

So while 3G mobile broadband is really taking off, it’s unlikely – as even Mr Russell concedes – that it will replace fixed broadband.

But the technology it really threatens is Wimax, commonly known as Wi-Fi on steroids.

BT’s new boss is talking of investing in Wimax to cover the big mobile gap in its portfolio.

But by the time that investment is in place Britain will be covered in High-Speed Downlink Packet Access (HSDPA) 3G networks – and they may just be fast enough to satisfy the appetite of most mobile broadband users.