Get the degree of an svg line.

gistfile1.js

/*
 * Draw text parallel to a line
 */

// Line
x1 = x(0)
y1 = y(0)
x2 = x(25)
y2 = y(30)

// Get the lines length
a = Math.abs(x2 - x1)
o = Math.abs(y2 - y1)
h = Math.sqrt(Math.pow(a, 2) + Math.pow(o, 2))

// Get the radian and put it as degree
deg = Math.asin(o / h) * (180 / Math.PI)

// Draw the line (hypotenuse)
el.append('line')
    .attr('x1', x1)
    .attr('y1', y1)
    .attr('x2', x2)
    .attr('y2', y2)

// Draw text parallel to the line
roi.append('text')
    .text('My rotated text')
    .attr('transform', 'translate(' + x2 + ',' + y2 + ')rotate(' + deg + ')')

@Calvein, this is faster and means you don't need a, o and h :)

var deg = Math.atan2(y2 - y1, x2 - x1)