[Feature] Set default legend icon for different series types

What problem does this feature solve?

I have not been able to figure out a way to do this. I think you should be able to declare a default icon type in the legend for each different series, for example: bar series: star icon line series: triangle icon I know that setting a different symbol on a line series will change the icon in the legend, but there is no way to do this for bar charts

What does the proposed API look like?

bar: { icon: 'star', }, line: { icon: 'triangle', }