fix crash on null color legend

[Fil]

(refs: #746 & #756)

[mbostock]

I don’t think this is the right place to fix this problem (because the issue of a scale being null is not unique to color legends). I think we need to fix it inside legendor exposeLegends?

For example, after #756, the if (isObject(scale)) is no longer sufficient to check whether a scale exists.

[mbostock]

Should this return undefined? Or should it throw an error?

Plot.legend({color: {}})

I guess it should throw an error (either unknown legend type or missing scale).

But the current behavior of exposeLegends is that it should return undefined.

[Fil]

I'm fine with both of these choices.

[mbostock]

Okay. I think my recommendation is that it behaves the same as Plot.legend({}) in the sense that there’s no scale defined, which is an error.

[mbostock]

This is pretty tricky, because there are three places where we’re using isObject as a proxy for “is a given scale defined?”

At the top-level when iterating over the passed in options:

plot/src/legends.js

Line 16 in 08118dd

if (isObject(scale)) { // e.g., ignore {color: "red"}

For a symbol scale to check whether there’s also a color scale:

plot/src/legends.js

Line 20 in 08118dd

const {fill, stroke = fill === undefined && isObject(options.color) ? "color" : undefined} = options;

In legendsSymbol to check whether there’s also a color scale:

plot/src/legends.js

Line 26 in 08118dd

key => isObject(options[key]) ? normalizeScale(key, options[key]) : null

It makes me wonder if #756 was a mistake since now the test for “is a scale defined” is more involved than testing whether the option exists.

[mbostock]

Superseded by #759.