Add info about NaNs and line/shape segments

[tbreloff]

should add to "input data"

[FelipeLema]

Since this is my fault, I might as well add a comment:

NaN provides a way to disjoint data. This comes in handy when grouping separate polygons that share some attribute.

using Plots; plotly()

function rectangle_from_coords(xb,yb,xt,yt)
    [
     xb yb
     xt yb
     xt yt
     xb yt
     xb yb
     NaN NaN
     ]
end

some_rects=[
       rectangle_from_coords(1 ,1 ,5 ,5 )
       rectangle_from_coords(10,10,15,15)
       ]
other_rects=[
       rectangle_from_coords(1 ,10,5 ,15)
       rectangle_from_coords(10,1 ,15,5 )

plot(some_rects[:,1], some_rects[:,2],label="some group")
plot!(other_rects[:,1], other_rects[:,2],label="other group")

[mkborregaard]

@FelipeLema if this is your fault, would you consider adding this in a PR? :-)