Confusing error message when constructing a line-charts axis

[dbrgn]

(Sorry if the terminology is sometimes a bit off, I'm still quite new to Elm.)

With the line-charts library and this module:

module Testcase exposing (makeAxis)

import LineChart.Axis as Axis
import LineChart.Axis.Line as AxisLine
import LineChart.Axis.Range as Range
import LineChart.Axis.Ticks as Ticks
import LineChart.Axis.Title as Title
import Time

type alias Measurement =
    { id : Int
    , sensorId : Maybe Int
    , temperature : Float
    , createdAt : Time.Posix
    }

makeAxis : Axis.Config Measurement msg
makeAxis =
    Axis.custom
        { title = Title.default ""
        , variable = Just << Time.posixToMillis << .createdAt
        , pixels = 500
        , range = Range.padded 20 20
        , axisLine = AxisLine.none
        , ticks = Ticks.time Time.utc 10
        }

...I get this error message:

It took me quite long to find out that the problem was that Time.posixToMillis returns an integer while the value should be a float. Changing the line to variable = Just << toFloat << Time.posixToMillis << .createdAt helped.

Issues:

• The Title.config msg value belonging to the title field in the record is yellow. Why? It doesn't seem to have anything to do with the problem.
• The variable value returns a Maybe Int. That's indicated in the error message. But it doesn't show that the expected value would be Maybe Float. Instead it shows some strange message related to Time.Posix and the createdAt field.

And not related to the error message:

• These are the docs for Axis.custom. Is there a way for me to find out what type the variable field should have? Maybe the source definition could be linked to from the docs? In this case the declaration of the Properties type alias would have helped.