Curved SVG path generator. Demo: http://jsbin.com/cimoqob

generator.js

function generateCurve (x1, y1, x2, y2, verticalBias) {
  
  var line = []
  var mx = x1 + (x2 - x1) / 2

  line.push('M', x1, y1)
  
  if (verticalBias) {
    var my = y1 + (y2 - y1) / 2
    line.push('C', x1, my, x2, my, x2, y2)
  } else {
    line.push('C', mx, y1, mx, y2, x2, y2)
  }
  
  return line.join(' ')
  
}

var path = document.querySelector('path')
path.setAttribute('d', generateCurve(0, 1, 200, 200))

markup.html

<svg width="1000" height="1000">
  <path d="" stroke="#000" stroke-width="2" fill="none" />
</svg>