Cross visual element consistency

[sehilyi]

Currently, we cannot support color consistency when color is used differently in two charts (e.g., nominal color for bar chart and quantitative color for heatmap). However, we can provide more flexible visual linkings by enabling to apply consistent color to different visual elements such as to axis label's background color.

Perhaps, we can support this in CompSpec, for example, by specifying target elements:

consistency: {
   color: {
      type: "same",   // we will change the term "same" to more proper one
      target: {
         secondary: {
            element: "axis-label", 
            property: "background"
         }
   }
}

When target is not provided, color is applied to cells, bars, or points by default.

[sehilyi]

This feature firstly mentioned in #35

[sehilyi]

Actually, we can provide color consistency between heatmap and bar chart by separating color to hue and saturation as mentioned in #35. However, this feature is still worth providing.

[sehilyi]

Lessen the spec level for the readability.

consistency: {
   color: {
      type: "shared",
      secondary_target: {
         element: "axis-label", 
         property: "background"
      }
}

[sehilyi]

THIS PART SHOULD BE MORE THOROUGHLY DETERMINED!