 layer through the <code>data</code> argument, columns of <code>data</code> can be used to specify various plot attributes such as <code>color</code>, etc. For example, with <code><a href='ly_points.html'>ly_points(..., data = iris, color = Species)</a></code>, the <code>Species</code> variable is used to determine how to color the points.  Here, <code>Species</code> is "mapped" to the <code>color</code> attribute.  Both continuous and categorical variables can be mapped.  In the case of continuous variable