Example of a D3js time series graph with X,Y crosshairs and a threshold line. Just copy the drawLineGraph function and call it with your data. The data shoud be an array of two element arrays. Something like:
var data = [
[new Date(2014, 01, 10), 404],
[new Date(2014, 01, 11), 123],
[new Date(2014, 01, 12), 666]
];
var warnLine = { lineValue: 200, label: 'my important threshold' };