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

[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.