Add translateY style parameter to legend + test

[schaumb]

No description provided.

[simzer]

I think this setter is a bit overcomplicated for the problem.
What we would like to be able to do is to set a transparency gradient for the label, if needed.
So why not simply give a Math::SegmentedFunction<double> transparencyGradient, Geom::Line gradientPosition parameters for DrawLabel and calculate the color gradient in DrawLabel from the calculated text color?

[schaumb]

because it's not just the label but the marker needs to. And that will be a duplication for the same gradient problem.

[simzer]

I see. It's unfortunate. Still could be more clean. I see three problems:

1. This class encapsulates three things (building of the ColorGradient, transforming the line, and setting it to the canvas). I think it would be better if it would have a singular purpose, building the ColorGradient.

2. (and 3.) How we build the color gradient is that we have an alpha/transparency gradient, and we get the color from outside. The class does not represent this, since it contains a colorgradient, and the transparency gradient is hard coded in the function.

I think a clearer and more generic approach would be to have a class, which stores an arbitrary alpha gradient and has a method which accepts a color and builds the ColorGradient.

Something like this:

/// legend draw:
fadeoutLine = markerWindowRect.leftSide()
fadeoutGradient = AlphaGradient({ 
  {0.0, 0.0}, 
  {fadeElementPercent, 1.0}, 
  {1.0 - fadeElementPercent, 1.0}, 
  {1.0, 0.0} 
})

// marker draw and label draw
canvas.setBrushGradient(transform(fadeoutLine), fadeoutGradient.toColorGradient(color))

[simzer]

Also, draw label is a more generic lower level utility class then legend draw. I think it's not a good practice to make the interface of the lower level class more cryptic to optimise code duplication in the higher level user class.