Chart: iPad, iPhone in spotlight as Apple preps earnings report

Apple’s earnings report on Tuesday afternoon will be closely watched as a gauge of the company’s momentum in the smartphone and tablet markets.

The company disappointed Wall Street in its last earnings report with iPhone sales that fell below analysts’ expectations, as the later-than-normal release of the latest iPhone, the 4S, caused consumers to hold back on purchases, as reflected by the dip in the iPhone in our chart above. But that pent-up demand could translate into some huge numbers in the latest report, which covers holiday sales and the October launch of the iPhone 4S.

Fortune’s Philip Elmer-Dewitt, who regularly polls analysts in advance of Apple’s earnings, says most are predicting iPhone unit sales north of 30 million, which would be an 84 percent increase over the 2010 holiday season.

The other big trends to watch will be iPad and Mac sales, given the relative weakness of the Windows PC market.

Microsoft’s Windows profits fell 11 percent in the company’s most recent quarter, as the company felt the effects of declining shipments caused in part by the flooding in Thailand impacting makers of PC hard drives.

A report released earlier today by the Pew Research Center found that the number of people owning a tablet nearly doubled between mid-December and early January, jumping from 10 percent to 19 percent. The iPad was presumably a big part of that growth, along with Amazon’s Kindle Fire.

Amazon reports earnings next week, although the Seattle company doesn’t traditionally disclose quarterly Kindle unit sales.