Last week we looked at copper -- dubbed Dr. Copper because it can be used to take the pulse of the economy's health -- and it's struggling price. Thankfully, the metal has recovered since then. But another commodity has been flashing before our eyes: Oil.

Oil prices have been in a serious decline, falling almost 20% over the past month. To the broader stock market's credit, it has held up pretty well given the decline in crude. Especially as the Energy Select Sector SPDR ETF (XLE) - Get Report has fallen more than 6% over the past 30 days. The decline in oil even led to a slew of oil-stock downgrades earlier this week. But oddly, the commodity's fall hasn't been alone. Interestingly, it's fallen alongside coffee prices.

Say what? It's true.

The Coffee Sub Index Total Return (JO) - Get Report is off almost 26% over the past year, while the U.S. Oil ETF (USO) - Get Report is down more than 25% in that same span. The two diverged in October for a bit, but then got back to tracking each other relatively closely.

In February 2017, the two became practically joined at the hip, with both commodities down about 10% since Feb. 1. There's no obvious explanation for the correlation, but traders and investors can keep an eye on it for clues. If coffee prices start to bottom while crude is still falling, it may be a sign oil is about bottom too. The opposite also applies.

So who benefits? Coffee companies like Starbucks (SBUX) - Get Report and Dunkin' Brands Group  (DNKN) - Get Report like lower coffee prices, while travel and leisure companies like Carnival Corp. (CCL) - Get Report  and Royal Caribbean (RCL) - Get Report also benefit from lower oil costs. Airlines benefit from lower oil too, perhaps one reason why analysts at Argus upgraded Southwest (LUV) - Get Report to buy from hold.

Argus' upgrade joins a number of others from this week, as analysts predicted as much as 30% upside for some airlines. Those that made the list include Delta Air Lines (DAL) - Get Report , United Continental (UAL) - Get Report , American Airlines (AAL) - Get Report and Southwest Airlines.

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Other Noteworthy Ratings Changes On Thursday

Actuant (ATU) was upgraded to equal-weight and assigned a $20 price target at Barclays.

Brinker International  (EAT) - Get Report was upgraded to market perform and assigned a $40 price target BMO.

CarMax (KMX) - Get Report was upgraded to outperform and assigned a $69 price target at RBC.

Magellan Midstream Partners (MMP) - Get Report was initiated with a hold rating and assigned a $73 price target at Jefferies.

Valero Energy (VLO) - Get Report was upgraded to buy and assigned an $80 price target at Jefferies.

Starbucks, Magellan and Southwest are holdings in Jim Cramer's Action Alerts PLUS Charitable Trust Portfolio. Want to be alerted before Cramer buys or sells SBUX, MMP and LUV? Learn more now.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.