Use link href, not link src

[danvk]

See https://stackoverflow.com/questions/48488444/how-to-set-annotations-when-using-the-candle-plotter

[coveralls]

Coverage remained the same at 90.195% when pulling dd719d9 on href-not-src into a2ac6f9 on master.