Documentation request for subplots

Some clarification of how to make and use subplots in the documentation or an example in
`XPlot/src/XPlot.Plotly/Tests/Subplots.fsx`
would be very helpful.

I have gotten this far, two axes are made but they are overlaid, and only one plot is visible.

```f#
#I "packages/XPlot.Plotly/lib/net45"
#I "packages/Newtonsoft.Json/lib/net45"
#r "XPlot.Plotly.dll"

open XPlot.Plotly
let layout =
Layout(
title = "Heatmap and Time Series Plot beside each other",
xaxis = Xaxis(),
yaxis = Yaxis(),
xaxis2 = Xaxis(),
yaxis2 = Yaxis()
)

let data = [ Heatmap(z=[ [ 1;3;4]; [6;4;2]; [8;5;4]]):> XPlot.Plotly.Graph.Trace
Scatter( y = [1; 3; 6], xaxis="x2", yaxis="y2") :> XPlot.Plotly.Graph.Trace ]

data
|> Chart.Plot
|> Chart.WithLayout layout
```