Dygraph charts are not displayed in WebView

WebView can't display charts with Dygraph library: http://dygraphs.com/gallery/#g/annotations

Lines are not visible: http://piccy.info/view3/6591894/dfeafc2dc2af39ed0760afcb9d508546/

The same page loaded in QtWeb browser (based on Webkit): http://piccy.info/view3/6591897/6216c3ba4fd54e3c908ecc0380c323bd/