After computing the png image, the `model.setData` call was treating
plotly's traces array (`data`) as if it were the bundle's `data`
property (`model.data`).

This was causing a malformed bundle to be saved in the notebook,
and causing only the image/png form to be restored on notebook load.

When opening a plotly figure from a file the `this.node.offsetWidth`
and `this.node.offsetHeight` values are 0, which causes an error in
`Plotly.toImage`.  Add guards to not call `Plotly.toImage` unless
width and height are positive numbers.