Device Pixel Ratio
By default the chart's canvas will use a 1:1 pixel ratio, unless the physical display has a higher pixel ratio (e.g. Retina displays).
For applications where a chart will be converted to a bitmap, or printed to a higher DPI medium it can be desirable to render the chart at a higher resolution than the default.
devicePixelRatio to a value other than 1 will force the canvas size to be scaled by that amount, relative to the container size. There should be no visible difference on screen; the difference will only be visible when the image is zoomed or printed.
||window.devicePixelRatio||Override the window's default devicePixelRatio.|