Chart.Line sometimes shows erroneous curves in the case of incomplete data

[tpetricek]

[Cross-posting from https://github.com/fslaborg/FsLab/issues/108]

In the following code, the two plots look quite different:

#load "packages/FsLab/FsLab.fsx"

open Deedle
 open FSharp.Data
 open XPlot.GoogleCharts
 open XPlot.GoogleCharts.Deedle

let wb = WorldBankData.GetDataContext()
 let cz = wb.Countries. Czech Republic .Indicators
 let eu = wb.Countries. European Union .Indicators
 let czforest = series cz. Forest area (% of land area) 
 let euforest = series eu. Forest area (% of land area) 

[ czforest.[1990 .. 1993]; euforest.[1990 .. 1993] ]
 |> Chart.Line
 |> Chart.WithOptions (Options(legend=Legend(position="bottom")))
 |> Chart.WithLabels ["CZ"; "EU"]

[ euforest.[1990 .. 1993]; czforest.[1990 .. 1993] ]
 |> Chart.Line
 |> Chart.WithOptions (Options(legend=Legend(position="bottom")))
 |> Chart.WithLabels ["EU"; "CZ"]

The difference seems to be due to missing data - there is no value for czforest.[1992]. My view is that the last version (interrupted curves) is the correct display of the data.