cross(a, b[, f])

[mbostock]

Proposed:

function pair(a, b) {
  return [a, b];
}

d3.cross = function(a, b, f) {
  var na = a.length, nb = b.length, c = new Array(na * nb), ia, ib, ic, va;
  if (f == null) f = pair;
  for (ia = ic = 0; ia < na; ++ia) for (va = a[ia], ib = 0; ib < nb; ++ib, ++ic) c[ic] = f(va, b[ib]);
  return c;
};

For example, given the following CSV data:

Year,Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec,J-D,D-N,DJF,MAM,JJA,SON
1880,-.30,-.21,-.18,-.27,-.14,-.29,-.24,-.08,-.17,-.16,-.19,-.22,-.20,***,***,-.20,-.20,-.17
1881,-.10,-.14,.01,-.03,-.04,-.28,-.07,-.03,-.09,-.20,-.26,-.16,-.12,-.12,-.15,-.02,-.13,-.19
1882,.09,.08,.01,-.20,-.18,-.25,-.11,.03,-.01,-.23,-.21,-.25,-.10,-.09,.00,-.12,-.11,-.15
1883,-.34,-.42,-.18,-.25,-.26,-.13,-.09,-.14,-.19,-.12,-.21,-.19,-.21,-.22,-.34,-.23,-.12,-.18
1884,-.18,-.13,-.36,-.36,-.32,-.38,-.35,-.27,-.24,-.22,-.30,-.30,-.28,-.28,-.17,-.35,-.33,-.25
1885,-.66,-.30,-.24,-.45,-.42,-.50,-.29,-.27,-.19,-.20,-.22,-.07,-.32,-.33,-.42,-.37,-.35,-.20
1886,-.43,-.46,-.41,-.29,-.27,-.39,-.16,-.31,-.19,-.25,-.26,-.25,-.31,-.29,-.32,-.33,-.29,-.23
1887,-.66,-.48,-.32,-.37,-.33,-.21,-.19,-.28,-.19,-.32,-.25,-.38,-.33,-.32,-.46,-.34,-.22,-.26
…

You could say:

d3.csv("temperatures.csv")
  .then(data => d3.cross(data.columns.slice(1, 13), data, (month, d) => ({
    date: d.Year + "-" + month,
    temperature: d[month]
  })))

[curran]

This would be awesome.

The term "melt" is used for a similar operation in the R world. See

• StackOverflow: how to pivot/unpivot (cast/melt) data frame?.
• Reshaping Data with the reshape Package (PDF) Great writeup on melting and casting by @hadley
• https://github.com/hadley/reshape - The R package where melt is implemented.

Here's a nice article on this topic by @eagereyes - Spreadsheet Thinking vs. Database Thinking.

Also, it would be interesting to consider how this would behave on sparse data. For example, would the operation let us get from this child mortality data from Gapminder to Lines with Missing Data?