Since IDC launched its Worldwide Quarterly Media Tablet and eReader Tracker in January, its coverage of the tablet market has been, shall we say, spotty.

Its tracking report for the first quarter of 2011 didn’t arrive until July, causing serious confusion among some high-profile tech writers who mistook it for a second-quarter report (see What slowdown in tablet sales?).

IDC’s tablet report for Q2 arrived Wednesday, two weeks before the start of Q4, and while it offered some worthwhile insights, it’s also full of holes. The chart above contains all the market share data we’ve been able to glean from IDC’s three tracking reports. We asked IDC’s press office on Wednesday if they could fill in the blanks; so far, no reply.

Meanwhile this much can be said: According to IDC …

• The worldwide tablet market is growing like gangbusters, with shipments up 88.9% from Q1 to Q2 and 303.8% year over year. IDC is now forecasting shipments of 62.5 million units in 2011, up from an earlier projection of 53.5 million
• Apple’s (AAPL) share, while down from Q3 2010, when it was nearly 87.4%, actually grew to 68.3% in Q2 (up from 65% in Q1) despite increased competition
• Research in Motion’s (RIMM) entered the market in Q2 with its PlayBook and grabbed a 4.9% share — at least in terms of units shipped
• The tablets running various versions of Google’s (GOOG) Android lost share, falling to 26.8% in Q2 (from 34% in Q1), and the news is likely to get worse before it gets better
• Hewlett-Packard’s (HPQ) probably bought a 4.7% share in Q3 with its firesale of $99 TouchPads, but given that HP has officially abandoned the TouchPad, that success is necessarily short-lived. By Q4 HP’s share will have fallen to zero