Make "loading" message be in same place and same aspect ratio as chart

[jrochkind]

Right now a "loading" message is seen towards the middle of the box.

When the data comes in, the "loading" message is removed, and the chart is added at the top of the box. This causes some jumping around after load.

We could make the loading message show up in same area that chart will eventually show up. And we could make it have the same aspect ratio as chart, using css aspect-ratio and our config, so it takes up the exact same space as the subsequent chart would.

If done right, this could be a lot less content moving around and resizing. But the kind of baroqueness of the JS makes it a bit tricky.

I had thought about wanting to do this before 9.0.0 release, but realize I don't have capacity right now, so am filing the ticket instead!