Allow plotly charts to be saved

[tdudgeon]

It will be useful be be able to save the scatter plot as a file that can be kept long term.
I believe this functionality is built in to plotly, just that the button to do it is not present in the toolbar?
Ideally save as PNG or SVG formats.

[OliverDudgeon]

I disabled this to clean up the UI a little. This can easily be turned back on but it should probably be configurable along with the other options.

[tdudgeon]

Let's make it visible by default for now.
We can configure it later.