function ratio( data, key1, key2 ) {
	var out;
	var i;
	var r;
	out = new Array( data.length );
	for ( i = 0; i < out.length; i++ ) {
		r = data[i][key1] / data[i][key2];
		out[ i ] = [ i, r ];
	}
	return out;
}
Computes the ratio of property values. The list consists of two-element tuples where the first element is the element index and the second element is the computed ratio.

@param {ObjectArray} data - input data
@param {string} key1 - numerator key
@param {string} key2 - denominator key
@returns {ArrayArray} array of tuples, where each tuple consists of the value index and the computed ratio