The value specification allows the mapping of a visual channel to a fixed constant value. This can be helpful in customizing the visualization, e.g., setting constant colors, re-arranging marks with fixed offsets, rotations, etc.

In the bookshelf keywords barchart example below, the book visual mark has its spine facing the back by default, and also has its pivot point in the center of the book's z dimension (depth) and thus occludes the x axis and labels.

The value specification can be used to assign fixed values to the book mark instances in order to rotate them 180 degrees with respect to the y axis (yrotation channel) and to add a fixed offset along the z dimension (zoffset, or zoffsetpct channels). Adding these fixed values rearranges the books so that their spines face the front and are offset towards the back to not occlude the labels.

Note that in this particular example, a fixed zoffset of 80 is specified, though it is also possible to use zoffsetpct of 0.5 which will achieve the same effect. The zoffsetpct is particularly useful if the actual depth of the book is unknown and the designer wants to offset the mark using a percentage of the depth. The similar channels xoffsetpct and yoffsetpct apply similarly to the width, and height, respectively.