While the Y axis is a Value type axis, the X axis can be a Category type axis or
a Value type axis. Using a Value axis, the data is treated as continuously
varying numerical data, and the marker is placed at a point along the axis which
varies according to its numerical value. Using a Category axis, the data is
treated as a sequence of non-numerical text labels, and the marker is placed at
a point along the axis according to its position in the sequence.