The plot_objective() function can take a (very) long time to run for problems like hart6 + two extra dimensions. Much longer than just hart6. Measured with the subjective "waiting for the plot to appear" stop watch in my head.

Adding a parameter with which to select the dimensions for which to make the plots and which dimensions to skip would be useful. If you are using the function several times to inspect your optimisation result you might want to only see a subset of all the dimensions.