Add doc example of transparency with Geom.ribbon

[IgorDouven]

According to the documentation, it should be possible to make Geom.ribbon colors transparent by adding something like Theme(lowlight_opacity=0.3). That does not appear to work however. This would be a great feature to have though.

[bjarthur]

indeed! there is no mention of lowlight_opacity in src/geom/ribbon.jl.

this would be a great first-time contribution. care to take a stab at fixing it? all you would need to do is follow the model here of how to programmatically specify opacity, and apply it here and here.

thanks for filing an issue.

[Mattriks]

From #611 (a workaround for now):

Theme(lowlight_color=c->RGBA{Float32}(c.r, c.g, c.b, 0.2)) 

[IgorDouven]

@Mattriks Thanks, this worked! (Would be worth adding to the documentation, even if it's only a workaround.)

@bjarthur I'd love to contribute, but this kind of problem is really beyond me (though I'm hoping to get to the point where it wouldn't be).

[bjarthur]

let's leave this open until it's fixed.

[bjarthur]

thinking on this more...permitting the user to specify both lowlight_opacity and lowlight_color simultaneously would need some special consideration to handle the case where the latter included an alpha channel. so the recommended course of action here i think is to just update the documentation to remove mention of the former and give an example of how to achieve the same effect with the latter.

[Mattriks]

I think fillopacity() will work for only svg e.g. compare

# The plot panel uses fillopacity(Theme.panel_opacity)
p = plot(x=[-2,2], y=[2,-2], 
    Theme(panel_fill=colorant"orange", panel_opacity=0.5)
)
draw(PNG(3.3inch,3.3inch), p)
draw(SVG(3.3inch,3.3inch), p)

whereas the lowlight_color function will work for svg and png.
lowlight_opacity looks like code that someone thought about, but wasn't implemented.

[tlnagy]

Grepping through the repo for lowlight_opacity only brings up its initialization in theme.jl so it's never used. I'm open to simply removing it (both from the docs and from the source) until we figure out a way to combine it with lowlight_color. It might be nice to throw a depwarn in the meantime.

[kleinschmidt]

As a general way of combining colors and alphas, why not use Colors.coloralpha? It just sets the alpha of whatever color you pass it, and promotes the type if necessary

julia> using Colors

julia> coloralpha(RGB(0.5, 0.5, 0.5))
RGBA{Float64}(0.5,0.5,0.5,1.0)

julia> coloralpha(RGB(0.5, 0.5, 0.5), 0.7)
RGBA{Float64}(0.5,0.5,0.5,0.7)

julia> coloralpha(RGBA(0.5, 0.5, 0.5, 1.0), 0.7)
RGBA{Float64}(0.5,0.5,0.5,0.7)

julia> c = Lab(RGB(0.2, 0.3, 0.4))
Lab{Float64}(31.579833005156537,-1.8639258539215886,-17.85083480738884)

julia> coloralpha(c, 0.7)
LabA{Float64}(31.579833005156537,-1.8639258539215886,-17.85083480738884,0.7)

That be a good basis for allowing an alpha aes, which I miss from ggplot...

[bjarthur]

@kleinschmidt have you seen #1175 ?

[Mattriks]

An alpha aes hopefully will be a future direction in Gadfly. But it would require a much bigger PR involving:

• implementing alpha as an aesthetic i.e. alpha=:variable
• Scale.alpha_discrete and Scale.alpha_continuous
• integration with Guides e.g. Guide.colorkey