Quarterly sales last year varied widely, from a low of 720,000 in June to a high of 6,890,000 in September following the release of the iPhone 3G.

But that’s nothing compared with the weird patterns that emerge from data collected by Net Applications, a Web metrics firm that tracks hits to its clients’ websites on a daily basis — a total of 160 million visitors per month.

The chart above (full size below the fold), created by Hank Vaccaro of Standard Analytics and posted last week on The Mac Observer’s Apple Finance Board, displays Net Applications’ iPhone data for the device’s first 18 months on the market. The first part of the chart is drawn from weekly data; the last part includes daily variations.

With the caveat that this graph represents how frequently iPhone owners are using the Web, not iPhone sales or market share, it still tells us quite a bit about the iPhone’s growth over the past year and a half.

First, by tracking the seven-day moving average (the red line in Vaccaro’s graph), we can see four clear spikes:

1. The initial release in June-July 2007
2. Holiday sales in Dec. 2007
3. The release of the iPhone 3G in July 2008
4. Holiday sales in Dec. 2008

We won’t know how strong sales were in December until Apple (AAPL) releases its quarterly earnings next week (Jan. 21 at 5 p.m. EST), but based on the magnitude of that last bump, they shouldn’t be too shabby. (Citibank’s Richard Gardner on Tuesday cut his 12-month price target for Apple shares from $153 to $132 based in part on what he called “soft 4CQ08 iPhone shipments,” but didn’t say where he got his data.)

And what about the strange saw-tooth pattern in the last part of the chart? That’s created by the daily variations in Web hits. On weekdays, the Internet tends to be dominated by office computers — mostly PCs running Windows. But on weekends, home users take over, and that includes a higher percentage of Macs, iPhones and other non-Windows devices. The variation is striking — as much as 40% in a single day.

But it all smooths out in the long run, as represented in Vaccaro’s chart by a thick black line, and the trend — so far — is an exponential rise.

See also:

• iPod touch use “exploded” Christmas day
• Apple’s Internet share registered strong gains in December
• iPhone Web share hits record 0.48%, up 58% in one month

Below the fold: Vaccaro’s chart, full size: