make an ECharts plot element

[miserman]

The only plot element uses Plotly, but it might be nice to implement an ECharts version to see if it handles some things better.

Impetus

• Plotly struggles with many traces, and its canvas variant doesn't seem to help
• Plotly struggles with certain style changes as part of highlighting -- can change line width, but changing marker size as well crashes

Stipulations

• this should be an option, rather than replacing the Plotly element
• it should support the same basic settings, such as trace limits and box plot visibility and variants