Dashboards: Add support for second y-axes in the line chart widget

[fietser28]

I would like to be able to have 2 (or more) y-axes on graphs. This would require a possibility to choose which y-axis a line is connected to in the UI.

more than 2 axes can be supported, but in practice this is not used very often. Maybe just start with 2 (left and right) for now if that is easier to implement.

Example is the recent battery dashboard I showed: The 2 graphs can be combined into 1. This will more clearly show the interaction between the graphs.

I think potly has support for this: https://plotly.com/javascript/multiple-axes/