Symbol should have sensible defaults about vega

[jheer]

Symbols already default to a size of 100 pixels if no size value is specified.

For all marks, stroke and fill default to transparent, by design. Setting either stroke or fill to (say) black by default would, while having the advantage of visibility, still require that one "unset" the default on many occasions. Given a default fill, symbols for instance might have to explicitly set fill to "none" or "transparent" and then explicitly set the stroke color. One could conjure more complicated schemes, but for simplicity we have chosen to avoid marktype-specific color defaults.

[hadley]

I understand the reasoning, but it does make debugging painful. I think a reasonable default would be that if both stroke and fill and not set, then one or the other should get a default.