refactor: `plotting-canvas` spans fullscreen independently

[TungVietLe]

As discussed in PR #326. Turn out it was the body padding of the CSS of my website. It might be a small addition. I would suggest adding:

canvas.style.pointerEvents = 'none'
canvas.style.top = '0px'
canvas.style.left = '0px'

so that the plotting-canvas would always span independently regardless of the client's CSS files.

Also, you are absolutely correct that it shifts and unshifts the canvas. Therefore, top and left equal to 0px would be sufficient.

[jeffhuang]

not a webgazer bug