Is “Something in the sand” a popular English phrase?

I’m interested in the line, “Raising tax rates on the wealthy is Obama’s line in the sand” in the following lead-copy of Washington Post’s (December 4) article titled “President Obama’s tough time taking a hard line on the fiscal cliff.”

“The president has done something that could make negotiations far more difficult. He has made it clear that raising tax rates on the wealthy is his line in the sand.”

I see the idiom, “Bury your head in the sand” in Cambridge online dictionary, and “Head in the sand” in Merriam-Webster online dictionary, but I cannot find “in the sand” as an idiom in both dictionaries. Oxford online dictionary registers only “the sands (of time) are running out” as a sand-related phrase.

On the other hand, Google Ngram shows consistent incidences of “in the sand” since cir 1840. I guess “his line in the sand” means “his statement (promise) unachievable like “a castle in the air.” Is “something (letter, record, plan, promise, structure) in the sand” a popular expression? If so, why isn’t it registered in major dictionaries as a simile of fragility, ephemerality, or easy-erasable like the title of Japanese popular song, "A love-letter writen on the sand"?

"X in the sand" doesn't really exist as a stand-alone.

A "line in the sand" sets a limit. Crossing that line will have consequences of some sort.

It is a very old idea. The Ramayana includes a line in the sand, drawn around a house to protect it. The Roman Laenas drew a line in the sand around King Antiochus, saying that Rome would declare war if he crossed the line without agreeing to leave Egypt. At the Alamo, Travis drew a line in the sand and invited those who wanted to refuse to surrender to cross it and join him.

According to Ngram, the phrase entered popular use in the closing years of the 18th Century.

To draw a line in the sand is, in the OED’s definition, ‘to establish a limit or boundary; to specify a level of tolerance or a point beyond which one will not go.’

The earliest citation is mid-twentieth century, and it is found quite frequently, at least in the UK, in political commentary.

"Line in the sand" seems to be well covered in the other answers. Head in the sand refers to Ostriches supposedly burying their head in the sand to avoid danger. Not true but it has become an idiom for not admitting there is a problem.