Using color of `Geom.hline` as aesthetic in Gadfly

Question

In Gadfly, I am trying to color horizontal lines based on data. But since color of Geom.hline is not an aesthetic, but only an argument, my approach fails. How can I pass color form the data to the color argument?

My attempt:

using DataFrames, Gadfly

test = DataFrame(y = [1,2], group = ["a", "b"])
p = plot(test, yintercept = :y, color=:group, Geom.hline)

Desired behavior:

Two horizontal lines with two different colors (colored by group).

Actual behavior:

Two horizontal lines with the same color, warning message The following aesthetics are mapped, but not used by any geometry: color.

Answer 1

You can pass a color argument to Geom.hline rather than to the plot function. As an example:

using DataFrames, Gadfly


test = DataFrame(y=[1, 2], group=["a", "b"])
p = plot(test, yintercept=:y, Geom.hline(color=["red", "blue"]))

This gives me the following result: In your case you would have to map your groups against colors