areaBetween draws incorrectly when used in multiple targets

[mleinartas-iseatz]

areaBetween() draws the two series passed to it as stacked lines with the bottom line set to invisible. Since there are actually 2 sets of stacked lines it ends up stacking 4 lines (with two of them being invisible) - the result is that the area drawn is between the outer band's lower and the inner band's lower, and between the inner band's upper and the outer band's lower. This also affects holtWintersConfidenceArea() as it uses areaBetween

[obfuscurity]

The areaBetween function is rather primitive and naive about the format it will accept series. Going to close this issue and track fixing the "robustness" of this function in #381.

Plus, it looks like @mleinartas-iseatz is probably @mleinart's old user account from iSeatz. 😄