color legend entry

[SimonDanisch]

Legend currently doesn't pick up an e.g. markercolor:

df = (
    x = 1:10,
    y = rand(10),
    attr = ["m $i" for i in 1:10],
)
draw(data(df) * mapping(:x, :y, marker=:attr) * visual(Scatter, color=:red))

This PR takes uses the color, if it's a single color:

if multiple colors are used, it defaults back to black:

draw(data(df) * (mapping(:x, :y, marker=:attr) * (visual(Scatter, color=:red, markersize=20) + visual(Scatter, color=:green, markersize=10))))

[greimel]

I don't fully understand what the code does. But what happens if you use color for grouping but there happens to be just one group? (In my opinion, the legend for marker should be black despite there being just one color.)

What about a more general solution that adds all keywords from visual to options (which are then passed on to legend_element)?

(Not sure if that's easily doable, though)

[piever]

I think this is not completely correct. What if one scatter layer has a color attribute, and another scatter layer has no color attribute, and is defaulting to the theme?

I think one should instead check that, for a given plottype, all the entries which use the marker scale (or whichever relevant scale is used in that legend group) also have a color attribute, and all the values agree. This may be not too hard to check, it should be all the entries where at least one of the label_attrs is in the keys of entry.primary.

I'm also not sure if it should only be done for color, or there are other attributes in visual that should be considered.

[piever]

I don't fully understand what the code does. But what happens if you use color for grouping but there happens to be just one group? (In my opinion, the legend for marker should be black despite there being just one color.)

I think this PR only considers colors that are given via visual. I confess I'm in two minds about this, I'm not completely sure which is a better default for the legend, whether it is the theme default or the values it encounters in visual (should they all agree). Do you have some intuition as to which is more natural?

Somehow, both things seem reasonable to me, and maybe it's not too bad to briefly change the theme to have red markers (maybe theme can be passed as a keyword to draw, and is merged to the default theme).

[jkrumbiegel]

this is fixed since 0.7