PlotlyJS GridLayout is broken for the first plot

The first plot seems to take up the entire plot area, with subsequent plots in the layout plotting over the top of it.
I expected this to be three evenly spaced plots stacked vertically:

```julia
plot(plot(rand(100)), plot(rand(100)), plot(rand(100)), layout=Plots.GridLayout(3, 1))
```

Edit: this is on master