add something like series.order

[leeoniya]

it's useful to have legend order differ from data order and rendering order (zindex).

when rendering, points should be in front of filled bars and those should be in front of line-areas.

legend/tooltip could be made dynamic based on some property like Last value, or other stat.

also some series maybe should be hidden from legend or graph (or tooltip).

maybe

series.order = {
  legend: 1,
  plot: null,  // auto or data order?
  tooltip: -1, // hide?
};