> Time should be on the x-axis. Independent variables on the x-axis and dependent variables on the y.
The graph is correct - the independent variables in this case are the number of items in the array (the x axis) and the library being tested (each line).
The dependent variable in this case is time - which is why it is on the y axis.
EDIT: To clarify, plotting 'seconds' over 'items per second' is needlessly confusing.